diff --git a/TODO.txt b/TODO.txt
index 6ca44b9..e2b7064 100644
--- a/TODO.txt
+++ b/TODO.txt
@@ -44,8 +44,10 @@ Milestone: v0.4.0
           higher level logic set the default from there.
     [ ] Write man page.
 
-[ ] Refactor MatrixErrorCreate() to take a custom message or NULL to
+[x] Refactor MatrixErrorCreate() to take a custom message or NULL to
     use the default. This will make debugging a lot easier.
+    [ ] Use detailed error messages for HTTP 500s at the minimum. Other
+        error messages might also be useful.
 [x] Make sure admin registration token is printed to log, not stdout.
     Unless they are the same, of course.
 [ ] Refactor Config to use j2s.
diff --git a/src/Matrix.c b/src/Matrix.c
index 802979e..39a7914 100644
--- a/src/Matrix.c
+++ b/src/Matrix.c
@@ -82,7 +82,7 @@ MatrixHttpHandler(HttpServerContext * context, void *argp)
     {
         HttpResponseHeader(context, "Content-Type", "application/json");
         HttpResponseStatus(context, HTTP_NOT_FOUND);
-        response = MatrixErrorCreate(M_NOT_FOUND);
+        response = MatrixErrorCreate(M_NOT_FOUND, NULL);
     }
 
     /*
@@ -115,7 +115,7 @@ MatrixHttpHandler(HttpServerContext * context, void *argp)
 }
 
 HashMap *
-MatrixErrorCreate(MatrixError errorArg)
+MatrixErrorCreate(MatrixError errorArg, char *msg)
 {
     HashMap *errorObj;
     char *errcode;
@@ -261,6 +261,11 @@ MatrixErrorCreate(MatrixError errorArg)
             return NULL;
     }
 
+    if (msg)
+    {
+        error = msg;
+    }
+
     errorObj = HashMapCreate();
     if (!errorObj)
     {
@@ -289,7 +294,7 @@ MatrixGetAccessToken(HttpServerContext * context, char **accessToken)
         if (strncmp(token, "Bearer ", 7) != 0)
         {
             HttpResponseStatus(context, HTTP_UNAUTHORIZED);
-            return MatrixErrorCreate(M_MISSING_TOKEN);
+            return MatrixErrorCreate(M_MISSING_TOKEN, NULL);
         }
 
         /* Seek past "Bearer" */
@@ -310,7 +315,7 @@ MatrixGetAccessToken(HttpServerContext * context, char **accessToken)
         if (!token)
         {
             HttpResponseStatus(context, HTTP_UNAUTHORIZED);
-            return MatrixErrorCreate(M_MISSING_TOKEN);
+            return MatrixErrorCreate(M_MISSING_TOKEN, NULL);
         }
     }
 
diff --git a/src/Routes/RouteChangePwd.c b/src/Routes/RouteChangePwd.c
index 90fee75..a0d1ccd 100644
--- a/src/Routes/RouteChangePwd.c
+++ b/src/Routes/RouteChangePwd.c
@@ -71,7 +71,7 @@ ROUTE_IMPL(RouteChangePwd, path, argp)
     {
         Log(LOG_ERR, "Password endpoint failed to lock configuration.");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        return MatrixErrorCreate(M_UNKNOWN);
+        return MatrixErrorCreate(M_UNKNOWN, NULL);
     }
 
     (void) path;
@@ -79,7 +79,7 @@ ROUTE_IMPL(RouteChangePwd, path, argp)
     if (HttpRequestMethodGet(args->context) != HTTP_POST)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_UNRECOGNIZED);
+        response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
         goto finish;
     }
 
@@ -93,7 +93,7 @@ ROUTE_IMPL(RouteChangePwd, path, argp)
     if (!request)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_NOT_JSON);
+        response = MatrixErrorCreate(M_NOT_JSON, NULL);
         goto finish;
     }
 
@@ -107,7 +107,7 @@ ROUTE_IMPL(RouteChangePwd, path, argp)
     if (uiaResult < 0)
     {
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        response = MatrixErrorCreate(M_UNKNOWN);
+        response = MatrixErrorCreate(M_UNKNOWN, NULL);
         goto finish;
     }
     else if (!uiaResult)
@@ -120,7 +120,7 @@ ROUTE_IMPL(RouteChangePwd, path, argp)
     {
         JsonFree(request);
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_BAD_JSON);
+        response = MatrixErrorCreate(M_BAD_JSON, NULL);
         goto finish;
     }
 
@@ -136,7 +136,7 @@ ROUTE_IMPL(RouteChangePwd, path, argp)
     if (!user)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_UNKNOWN_TOKEN);
+        response = MatrixErrorCreate(M_UNKNOWN_TOKEN, NULL);
         goto finish;
     }
 
diff --git a/src/Routes/RouteConfig.c b/src/Routes/RouteConfig.c
index a3b7142..194a147 100644
--- a/src/Routes/RouteConfig.c
+++ b/src/Routes/RouteConfig.c
@@ -52,14 +52,14 @@ ROUTE_IMPL(RouteConfig, path, argp)
     if (!user)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_UNKNOWN_TOKEN);
+        response = MatrixErrorCreate(M_UNKNOWN_TOKEN, NULL);
         goto finish;
     }
 
     if (!(UserGetPrivileges(user) & USER_CONFIG))
     {
         HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-        response = MatrixErrorCreate(M_FORBIDDEN);
+        response = MatrixErrorCreate(M_FORBIDDEN, NULL);
         goto finish;
     }
 
@@ -68,7 +68,7 @@ ROUTE_IMPL(RouteConfig, path, argp)
     {
         Log(LOG_ERR, "Config endpoint failed to lock configuration.");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        response = MatrixErrorCreate(M_UNKNOWN);
+        response = MatrixErrorCreate(M_UNKNOWN, NULL);
         goto finish;
     }
 
@@ -82,7 +82,7 @@ ROUTE_IMPL(RouteConfig, path, argp)
             if (!request)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_NOT_JSON);
+                response = MatrixErrorCreate(M_NOT_JSON, NULL);
                 break;
             }
 
@@ -90,7 +90,7 @@ ROUTE_IMPL(RouteConfig, path, argp)
             if (!newConf)
             {
                 HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-                response = MatrixErrorCreate(M_UNKNOWN);
+                response = MatrixErrorCreate(M_UNKNOWN, NULL);
                 break;
             }
 
@@ -108,14 +108,13 @@ ROUTE_IMPL(RouteConfig, path, argp)
                 else
                 {
                     HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-                    response = MatrixErrorCreate(M_UNKNOWN);
+                    response = MatrixErrorCreate(M_UNKNOWN, NULL);
                 }
             }
             else
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                /* TODO: Attach newConf->err as message */
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, newConf->err);
             }
 
             ConfigFree(newConf);
@@ -125,7 +124,7 @@ ROUTE_IMPL(RouteConfig, path, argp)
             /* TODO: Support incremental changes to the config */
         default:
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_UNRECOGNIZED);
+            response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
             break;
     }
 
diff --git a/src/Routes/RouteDeactivate.c b/src/Routes/RouteDeactivate.c
index 309f8c2..09538f8 100644
--- a/src/Routes/RouteDeactivate.c
+++ b/src/Routes/RouteDeactivate.c
@@ -53,14 +53,14 @@ ROUTE_IMPL(RouteDeactivate, path, argp)
     {
         Log(LOG_ERR, "Deactivate endpoint failed to lock configuration.");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        response = MatrixErrorCreate(M_UNKNOWN);
+        response = MatrixErrorCreate(M_UNKNOWN, NULL);
         goto finish;
     }
 
     if (HttpRequestMethodGet(args->context) != HTTP_POST)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_UNRECOGNIZED);
+        response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
         goto finish;
     }
 
@@ -68,7 +68,7 @@ ROUTE_IMPL(RouteDeactivate, path, argp)
     if (!request)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_NOT_JSON);
+        response = MatrixErrorCreate(M_NOT_JSON, NULL);
         goto finish;
     }
 
@@ -96,7 +96,7 @@ ROUTE_IMPL(RouteDeactivate, path, argp)
     if (uiaResult < 0)
     {
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        response = MatrixErrorCreate(M_UNKNOWN);
+        response = MatrixErrorCreate(M_UNKNOWN, NULL);
         goto finish;
     }
     else if (!uiaResult)
@@ -122,14 +122,14 @@ ROUTE_IMPL(RouteDeactivate, path, argp)
     if (!user)
     {
         HttpResponseStatus(args->context, HTTP_UNAUTHORIZED);
-        response = MatrixErrorCreate(M_UNKNOWN_TOKEN);
+        response = MatrixErrorCreate(M_UNKNOWN_TOKEN, NULL);
         goto finish;
     }
 
     if (!UserDeleteTokens(user, NULL) || !UserDeactivate(user))
     {
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        response = MatrixErrorCreate(M_UNKNOWN);
+        response = MatrixErrorCreate(M_UNKNOWN, NULL);
         goto finish;
     }
 
diff --git a/src/Routes/RouteFilter.c b/src/Routes/RouteFilter.c
index aeee5e6..5b79901 100644
--- a/src/Routes/RouteFilter.c
+++ b/src/Routes/RouteFilter.c
@@ -73,14 +73,14 @@ ROUTE_IMPL(RouteFilter, path, argp)
     {
         /* Should be impossible */
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        return MatrixErrorCreate(M_UNKNOWN);
+        return MatrixErrorCreate(M_UNKNOWN, NULL);
     }
 
     serverName = GetServerName(db);
     if (!serverName)
     {
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        response = MatrixErrorCreate(M_UNKNOWN);
+        response = MatrixErrorCreate(M_UNKNOWN, NULL);
         goto finish;
     }
 
@@ -88,14 +88,14 @@ ROUTE_IMPL(RouteFilter, path, argp)
     if (!id)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_INVALID_PARAM);
+        response = MatrixErrorCreate(M_INVALID_PARAM, NULL);
         goto finish;
     }
 
     if (!StrEquals(id->server, serverName))
     {
         HttpResponseStatus(args->context, HTTP_UNAUTHORIZED);
-        response = MatrixErrorCreate(M_UNAUTHORIZED);
+        response = MatrixErrorCreate(M_UNAUTHORIZED, NULL);
         goto finish;
     }
 
@@ -109,14 +109,14 @@ ROUTE_IMPL(RouteFilter, path, argp)
     if (!user)
     {
         HttpResponseStatus(args->context, HTTP_UNAUTHORIZED);
-        response = MatrixErrorCreate(M_UNKNOWN_TOKEN);
+        response = MatrixErrorCreate(M_UNKNOWN_TOKEN, NULL);
         goto finish;
     }
 
     if (!StrEquals(id->localpart, UserGetName(user)))
     {
         HttpResponseStatus(args->context, HTTP_UNAUTHORIZED);
-        response = MatrixErrorCreate(M_INVALID_PARAM);
+        response = MatrixErrorCreate(M_INVALID_PARAM, NULL);
         goto finish;
     }
 
@@ -127,7 +127,7 @@ ROUTE_IMPL(RouteFilter, path, argp)
         if (!ref)
         {
             HttpResponseStatus(args->context, HTTP_NOT_FOUND);
-            response = MatrixErrorCreate(M_NOT_FOUND);
+            response = MatrixErrorCreate(M_NOT_FOUND, NULL);
             goto finish;
         }
 
@@ -147,14 +147,13 @@ ROUTE_IMPL(RouteFilter, path, argp)
         if (!request)
         {
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_NOT_JSON);
+            response = MatrixErrorCreate(M_NOT_JSON, NULL);
             goto finish;
         }
 
         if (!FilterFromJson(request, &filter, &parseErr))
         {
-            /* TODO: send parseErr to client */
-            response = MatrixErrorCreate(M_BAD_JSON);
+            response = MatrixErrorCreate(M_BAD_JSON, parseErr);
             goto finish;
         }
 
@@ -163,7 +162,7 @@ ROUTE_IMPL(RouteFilter, path, argp)
         if (!filterId)
         {
             HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-            response = MatrixErrorCreate(M_UNKNOWN);
+            response = MatrixErrorCreate(M_UNKNOWN, NULL);
             goto finish;
         }
 
@@ -172,7 +171,7 @@ ROUTE_IMPL(RouteFilter, path, argp)
         {
             Free(filterId);
             HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-            response = MatrixErrorCreate(M_UNKNOWN);
+            response = MatrixErrorCreate(M_UNKNOWN, NULL);
             goto finish;
         }
 
@@ -191,7 +190,7 @@ ROUTE_IMPL(RouteFilter, path, argp)
     else
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_UNRECOGNIZED);
+        response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
     }
 
 finish:
diff --git a/src/Routes/RouteLogin.c b/src/Routes/RouteLogin.c
index 4e8d768..e73f599 100644
--- a/src/Routes/RouteLogin.c
+++ b/src/Routes/RouteLogin.c
@@ -63,7 +63,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
     {
         Log(LOG_ERR, "Login endpoint failed to lock configuration.");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        return MatrixErrorCreate(M_UNKNOWN);
+        return MatrixErrorCreate(M_UNKNOWN, NULL);
     }
 
     (void) path;
@@ -84,7 +84,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
             if (!request)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_NOT_JSON);
+                response = MatrixErrorCreate(M_NOT_JSON, NULL);
                 break;
             }
 
@@ -92,14 +92,14 @@ ROUTE_IMPL(RouteLogin, path, argp)
             if (!val)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_MISSING_PARAM);
+                response = MatrixErrorCreate(M_MISSING_PARAM, NULL);
                 break;
             }
 
             if (JsonValueType(val) != JSON_STRING)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 break;
             }
 
@@ -107,7 +107,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
             if (!StrEquals(type, "m.login.password"))
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_UNRECOGNIZED);
+                response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
                 break;
             }
 
@@ -115,14 +115,14 @@ ROUTE_IMPL(RouteLogin, path, argp)
             if (!val)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_MISSING_PARAM);
+                response = MatrixErrorCreate(M_MISSING_PARAM, NULL);
                 break;
             }
 
             if (JsonValueType(val) != JSON_OBJECT)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 break;
             }
 
@@ -132,14 +132,14 @@ ROUTE_IMPL(RouteLogin, path, argp)
             if (!val)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_MISSING_PARAM);
+                response = MatrixErrorCreate(M_MISSING_PARAM, NULL);
                 break;
             }
 
             if (JsonValueType(val) != JSON_STRING)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 break;
             }
 
@@ -147,7 +147,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
             if (!StrEquals(type, "m.id.user"))
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_UNRECOGNIZED);
+                response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
                 break;
             }
 
@@ -155,14 +155,14 @@ ROUTE_IMPL(RouteLogin, path, argp)
             if (!val)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_MISSING_PARAM);
+                response = MatrixErrorCreate(M_MISSING_PARAM, NULL);
                 break;
             }
 
             if (JsonValueType(val) != JSON_STRING)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 break;
             }
 
@@ -170,7 +170,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
             if (!userId)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 break;
             }
 
@@ -178,7 +178,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
                 || !UserExists(db, userId->localpart))
             {
                 HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-                response = MatrixErrorCreate(M_FORBIDDEN);
+                response = MatrixErrorCreate(M_FORBIDDEN, NULL);
                 break;
             }
 
@@ -188,7 +188,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
                 if (JsonValueType(val) != JSON_STRING)
                 {
                     HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                    response = MatrixErrorCreate(M_BAD_JSON);
+                    response = MatrixErrorCreate(M_BAD_JSON, NULL);
                     break;
                 }
 
@@ -201,7 +201,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
                 if (JsonValueType(val) != JSON_STRING)
                 {
                     HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                    response = MatrixErrorCreate(M_BAD_JSON);
+                    response = MatrixErrorCreate(M_BAD_JSON, NULL);
                     break;
                 }
 
@@ -212,14 +212,14 @@ ROUTE_IMPL(RouteLogin, path, argp)
             if (!val)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_MISSING_PARAM);
+                response = MatrixErrorCreate(M_MISSING_PARAM, NULL);
                 break;
             }
 
             if (JsonValueType(val) != JSON_STRING)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 break;
             }
 
@@ -231,7 +231,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
                 if (JsonValueType(val) != JSON_BOOLEAN)
                 {
                     HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                    response = MatrixErrorCreate(M_BAD_JSON);
+                    response = MatrixErrorCreate(M_BAD_JSON, NULL);
                     break;
                 }
 
@@ -243,7 +243,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
             if (!user)
             {
                 HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-                response = MatrixErrorCreate(M_FORBIDDEN);
+                response = MatrixErrorCreate(M_FORBIDDEN, NULL);
                 break;
             }
 
@@ -252,7 +252,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
                 UserUnlock(user);
 
                 HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-                response = MatrixErrorCreate(M_USER_DEACTIVATED);
+                response = MatrixErrorCreate(M_USER_DEACTIVATED, NULL);
                 break;
             }
 
@@ -264,7 +264,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
                 UserUnlock(user);
 
                 HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-                response = MatrixErrorCreate(M_FORBIDDEN);
+                response = MatrixErrorCreate(M_FORBIDDEN, NULL);
                 break;
             }
 
@@ -301,7 +301,7 @@ ROUTE_IMPL(RouteLogin, path, argp)
             break;
         default:
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_UNRECOGNIZED);
+            response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
             break;
     }
 
diff --git a/src/Routes/RouteLogout.c b/src/Routes/RouteLogout.c
index ecaf4bd..c469ee1 100644
--- a/src/Routes/RouteLogout.c
+++ b/src/Routes/RouteLogout.c
@@ -45,7 +45,7 @@ ROUTE_IMPL(RouteLogout, path, argp)
     if (HttpRequestMethodGet(args->context) != HTTP_POST)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        return MatrixErrorCreate(M_UNRECOGNIZED);
+        return MatrixErrorCreate(M_UNRECOGNIZED, NULL);
     }
 
     response = MatrixGetAccessToken(args->context, &tokenstr);
@@ -58,7 +58,7 @@ ROUTE_IMPL(RouteLogout, path, argp)
     if (!user)
     {
         HttpResponseStatus(args->context, HTTP_UNAUTHORIZED);
-        return MatrixErrorCreate(M_UNKNOWN_TOKEN);
+        return MatrixErrorCreate(M_UNKNOWN_TOKEN, NULL);
     }
 
     if (ArraySize(path) == 1)
@@ -66,7 +66,7 @@ ROUTE_IMPL(RouteLogout, path, argp)
         if (!StrEquals(ArrayGet(path, 0), "all"))
         {
             HttpResponseStatus(args->context, HTTP_NOT_FOUND);
-            response = MatrixErrorCreate(M_NOT_FOUND);
+            response = MatrixErrorCreate(M_NOT_FOUND, NULL);
             goto finish;
         }
 
@@ -75,7 +75,7 @@ ROUTE_IMPL(RouteLogout, path, argp)
             /* If we can't delete all of our tokens, then something is
              * wrong. */
             HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-            response = MatrixErrorCreate(M_UNKNOWN);
+            response = MatrixErrorCreate(M_UNKNOWN, NULL);
             goto finish;
         }
         response = HashMapCreate();
@@ -85,7 +85,7 @@ ROUTE_IMPL(RouteLogout, path, argp)
         if (!UserDeleteToken(user, tokenstr))
         {
             HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-            response = MatrixErrorCreate(M_UNKNOWN);
+            response = MatrixErrorCreate(M_UNKNOWN, NULL);
             goto finish;
         }
 
diff --git a/src/Routes/RoutePrivileges.c b/src/Routes/RoutePrivileges.c
index 1d7bbe3..f0273ad 100644
--- a/src/Routes/RoutePrivileges.c
+++ b/src/Routes/RoutePrivileges.c
@@ -49,14 +49,14 @@ ROUTE_IMPL(RoutePrivileges, path, argp)
     if (!user)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_UNKNOWN_TOKEN);
+        response = MatrixErrorCreate(M_UNKNOWN_TOKEN, NULL);
         goto finish;
     }
 
     if (!(UserGetPrivileges(user) & USER_GRANT_PRIVILEGES))
     {
         HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-        response = MatrixErrorCreate(M_FORBIDDEN);
+        response = MatrixErrorCreate(M_FORBIDDEN, NULL);
         goto finish;
     }
 
@@ -69,7 +69,7 @@ ROUTE_IMPL(RoutePrivileges, path, argp)
         if (!user)
         {
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_INVALID_PARAM);
+            response = MatrixErrorCreate(M_INVALID_PARAM, NULL);
             goto finish;
         }
     }
@@ -83,7 +83,7 @@ ROUTE_IMPL(RoutePrivileges, path, argp)
             if (!request)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_NOT_JSON);
+                response = MatrixErrorCreate(M_NOT_JSON, NULL);
                 break;
             }
 
@@ -91,7 +91,7 @@ ROUTE_IMPL(RoutePrivileges, path, argp)
             if (!val || JsonValueType(val) != JSON_ARRAY)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 break;
             }
 
@@ -117,7 +117,7 @@ ROUTE_IMPL(RoutePrivileges, path, argp)
             if (!UserSetPrivileges(user, privileges))
             {
                 HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-                response = MatrixErrorCreate(M_UNKNOWN);
+                response = MatrixErrorCreate(M_UNKNOWN, NULL);
                 break;
             }
 
@@ -128,7 +128,7 @@ ROUTE_IMPL(RoutePrivileges, path, argp)
             break;
         default:
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_UNRECOGNIZED);
+            response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
             goto finish;
             break;
     }
diff --git a/src/Routes/RouteProcControl.c b/src/Routes/RouteProcControl.c
index 4fad31b..adc8758 100644
--- a/src/Routes/RouteProcControl.c
+++ b/src/Routes/RouteProcControl.c
@@ -48,14 +48,14 @@ ROUTE_IMPL(RouteProcControl, path, argp)
     if (!user)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_UNKNOWN_TOKEN);
+        response = MatrixErrorCreate(M_UNKNOWN_TOKEN, NULL);
         goto finish;
     }
 
     if (!(UserGetPrivileges(user) & USER_PROC_CONTROL))
     {
         HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-        response = MatrixErrorCreate(M_FORBIDDEN);
+        response = MatrixErrorCreate(M_FORBIDDEN, NULL);
         goto finish;
     }
 
@@ -73,7 +73,7 @@ ROUTE_IMPL(RouteProcControl, path, argp)
             else
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_UNRECOGNIZED);
+                response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
                 goto finish;
             }
             break;
@@ -90,12 +90,12 @@ ROUTE_IMPL(RouteProcControl, path, argp)
             else
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_UNRECOGNIZED);
+                response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
                 goto finish;
             }
         default:
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_UNRECOGNIZED);
+            response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
             goto finish;
             break;
     }
diff --git a/src/Routes/RouteRefresh.c b/src/Routes/RouteRefresh.c
index 04de9e9..624029d 100644
--- a/src/Routes/RouteRefresh.c
+++ b/src/Routes/RouteRefresh.c
@@ -56,21 +56,21 @@ ROUTE_IMPL(RouteRefresh, path, argp)
     if (HttpRequestMethodGet(args->context) != HTTP_POST)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        return MatrixErrorCreate(M_UNRECOGNIZED);
+        return MatrixErrorCreate(M_UNRECOGNIZED, NULL);
     }
 
     request = JsonDecode(HttpServerStream(args->context));
     if (!request)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        return MatrixErrorCreate(M_NOT_JSON);
+        return MatrixErrorCreate(M_NOT_JSON, NULL);
     }
 
     val = HashMapGet(request, "refresh_token");
     if (!val || JsonValueType(val) != JSON_STRING)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_BAD_JSON);
+        response = MatrixErrorCreate(M_BAD_JSON, NULL);
         goto finish;
     }
 
@@ -82,7 +82,7 @@ ROUTE_IMPL(RouteRefresh, path, argp)
     if (!rtRef)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_UNKNOWN_TOKEN);
+        response = MatrixErrorCreate(M_UNKNOWN_TOKEN, NULL);
         goto finish;
     }
 
@@ -96,7 +96,7 @@ ROUTE_IMPL(RouteRefresh, path, argp)
             refreshToken);
         Log(LOG_WARNING, "This refresh token will be deleted.");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        response = MatrixErrorCreate(M_UNKNOWN);
+        response = MatrixErrorCreate(M_UNKNOWN, NULL);
 
         DbUnlock(db, rtRef);
         DbDelete(db, 3, "tokens", "refresh", refreshToken);
@@ -114,7 +114,7 @@ ROUTE_IMPL(RouteRefresh, path, argp)
             oldAccessToken);
         Log(LOG_WARNING, "This access token will be deleted.");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        response = MatrixErrorCreate(M_UNKNOWN);
+        response = MatrixErrorCreate(M_UNKNOWN, NULL);
 
         DbUnlock(db, rtRef);
         DbDelete(db, 3, "tokens", "refresh", refreshToken);
diff --git a/src/Routes/RouteRegister.c b/src/Routes/RouteRegister.c
index d9a366a..d8e4103 100644
--- a/src/Routes/RouteRegister.c
+++ b/src/Routes/RouteRegister.c
@@ -83,7 +83,7 @@ ROUTE_IMPL(RouteRegister, path, argp)
     {
         Log(LOG_ERR, "Registration endpoint failed to lock configuration.");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        return MatrixErrorCreate(M_UNKNOWN);
+        return MatrixErrorCreate(M_UNKNOWN, NULL);
     }
 
     if (ArraySize(path) == 0)
@@ -91,7 +91,7 @@ ROUTE_IMPL(RouteRegister, path, argp)
         if (HttpRequestMethodGet(args->context) != HTTP_POST)
         {
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_UNRECOGNIZED);
+            response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
             goto end;
         }
 
@@ -99,7 +99,7 @@ ROUTE_IMPL(RouteRegister, path, argp)
         if (!request)
         {
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_NOT_JSON);
+            response = MatrixErrorCreate(M_NOT_JSON, NULL);
             goto end;
         }
 
@@ -109,7 +109,7 @@ ROUTE_IMPL(RouteRegister, path, argp)
             if (JsonValueType(val) != JSON_STRING)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 goto finish;
             }
             username = StrDuplicate(JsonValueAsString(val));
@@ -117,14 +117,14 @@ ROUTE_IMPL(RouteRegister, path, argp)
             if (!UserValidate(username, config->serverName))
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_INVALID_USERNAME);
+                response = MatrixErrorCreate(M_INVALID_USERNAME, NULL);
                 goto finish;
             }
 
             if (UserExists(db, username))
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_USER_IN_USE);
+                response = MatrixErrorCreate(M_USER_IN_USE, NULL);
                 goto finish;
             }
         }
@@ -144,7 +144,7 @@ ROUTE_IMPL(RouteRegister, path, argp)
         if (uiaResult < 0)
         {
             HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-            response = MatrixErrorCreate(M_UNKNOWN);
+            response = MatrixErrorCreate(M_UNKNOWN, NULL);
             goto finish;
         }
         else if (!uiaResult)
@@ -159,7 +159,7 @@ ROUTE_IMPL(RouteRegister, path, argp)
         if (kind && !StrEquals(kind, "user"))
         {
             HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-            response = MatrixErrorCreate(M_INVALID_PARAM);
+            response = MatrixErrorCreate(M_INVALID_PARAM, NULL);
             goto finish;
         }
 
@@ -167,14 +167,14 @@ ROUTE_IMPL(RouteRegister, path, argp)
         if (!val)
         {
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_MISSING_PARAM);
+            response = MatrixErrorCreate(M_MISSING_PARAM, NULL);
             goto finish;
         }
 
         if (JsonValueType(val) != JSON_STRING)
         {
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_BAD_JSON);
+            response = MatrixErrorCreate(M_BAD_JSON, NULL);
             goto finish;
         }
 
@@ -186,7 +186,7 @@ ROUTE_IMPL(RouteRegister, path, argp)
             if (JsonValueType(val) != JSON_STRING)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 goto finish;
             }
 
@@ -199,7 +199,7 @@ ROUTE_IMPL(RouteRegister, path, argp)
             if (JsonValueType(val) != JSON_BOOLEAN)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 goto finish;
             }
 
@@ -212,7 +212,7 @@ ROUTE_IMPL(RouteRegister, path, argp)
             if (JsonValueType(val) != JSON_STRING)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 goto finish;
             }
 
@@ -225,7 +225,7 @@ ROUTE_IMPL(RouteRegister, path, argp)
             if (JsonValueType(val) != JSON_BOOLEAN)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_BAD_JSON);
+                response = MatrixErrorCreate(M_BAD_JSON, NULL);
                 goto finish;
             }
 
@@ -311,12 +311,12 @@ finish:
             if (!username)
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_MISSING_PARAM);
+                response = MatrixErrorCreate(M_MISSING_PARAM, NULL);
             }
             else if (!UserValidate(username, config->serverName))
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_INVALID_USERNAME);
+                response = MatrixErrorCreate(M_INVALID_USERNAME, NULL);
             }
             else if (!UserExists(db, username))
             {
@@ -326,13 +326,13 @@ finish:
             else
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_USER_IN_USE);
+                response = MatrixErrorCreate(M_USER_IN_USE, NULL);
             }
         }
         else
         {
             HttpResponseStatus(args->context, HTTP_NOT_FOUND);
-            response = MatrixErrorCreate(M_UNRECOGNIZED);
+            response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
         }
     }
 
diff --git a/src/Routes/RouteRequestToken.c b/src/Routes/RouteRequestToken.c
index 33970ca..46bba31 100644
--- a/src/Routes/RouteRequestToken.c
+++ b/src/Routes/RouteRequestToken.c
@@ -38,21 +38,21 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
     if (HttpRequestMethodGet(args->context) != HTTP_POST)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        return MatrixErrorCreate(M_UNRECOGNIZED);
+        return MatrixErrorCreate(M_UNRECOGNIZED, NULL);
     }
 
     request = JsonDecode(HttpServerStream(args->context));
     if (!request)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        return MatrixErrorCreate(M_NOT_JSON);
+        return MatrixErrorCreate(M_NOT_JSON, NULL);
     }
 
     val = HashMapGet(request, "client_secret");
     if (!val || JsonValueType(val) != JSON_STRING)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_BAD_JSON);
+        response = MatrixErrorCreate(M_BAD_JSON, NULL);
         goto finish;
     }
 
@@ -60,7 +60,7 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
     if (strlen(str) > 255 || StrBlank(str))
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_BAD_JSON);
+        response = MatrixErrorCreate(M_BAD_JSON, NULL);
         goto finish;
     }
 
@@ -68,7 +68,7 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
     if (!val || JsonValueType(val) != JSON_INTEGER)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_BAD_JSON);
+        response = MatrixErrorCreate(M_BAD_JSON, NULL);
         goto finish;
     }
 
@@ -76,7 +76,7 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
     if (val && JsonValueType(val) != JSON_STRING)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_BAD_JSON);
+        response = MatrixErrorCreate(M_BAD_JSON, NULL);
         goto finish;
     }
 
@@ -84,7 +84,7 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
     if (val && JsonValueType(val) != JSON_STRING)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_BAD_JSON);
+        response = MatrixErrorCreate(M_BAD_JSON, NULL);
         goto finish;
     }
 
@@ -92,7 +92,7 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
     if (val && JsonValueType(val) != JSON_STRING)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_BAD_JSON);
+        response = MatrixErrorCreate(M_BAD_JSON, NULL);
         goto finish;
     }
 
@@ -102,7 +102,7 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
         if (val && JsonValueType(val) != JSON_STRING)
         {
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_BAD_JSON);
+            response = MatrixErrorCreate(M_BAD_JSON, NULL);
             goto finish;
         }
     }
@@ -112,7 +112,7 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
         if (val && JsonValueType(val) != JSON_STRING)
         {
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_BAD_JSON);
+            response = MatrixErrorCreate(M_BAD_JSON, NULL);
             goto finish;
         }
 
@@ -120,7 +120,7 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
         if (strlen(str) != 2)
         {
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_BAD_JSON);
+            response = MatrixErrorCreate(M_BAD_JSON, NULL);
             goto finish;
         }
 
@@ -128,7 +128,7 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
         if (val && JsonValueType(val) != JSON_STRING)
         {
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_BAD_JSON);
+            response = MatrixErrorCreate(M_BAD_JSON, NULL);
             goto finish;
         }
     }
@@ -136,12 +136,12 @@ ROUTE_IMPL(RouteRequestToken, path, argp)
     {
         /* Should not be possible */
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        response = MatrixErrorCreate(M_UNKNOWN);
+        response = MatrixErrorCreate(M_UNKNOWN, NULL);
         goto finish;
     }
 
     HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-    response = MatrixErrorCreate(M_THREEPID_DENIED);
+    response = MatrixErrorCreate(M_THREEPID_DENIED, NULL);
 
 finish:
     JsonFree(request);
diff --git a/src/Routes/RouteStaticResources.c b/src/Routes/RouteStaticResources.c
index a89b018..5d59705 100644
--- a/src/Routes/RouteStaticResources.c
+++ b/src/Routes/RouteStaticResources.c
@@ -35,7 +35,7 @@ ROUTE_IMPL(RouteStaticResources, path, argp)
     {
         /* Should be impossible */
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        return MatrixErrorCreate(M_UNKNOWN);
+        return MatrixErrorCreate(M_UNKNOWN, NULL);
     }
 
     if (StrEquals(res, "js"))
@@ -148,7 +148,7 @@ ROUTE_IMPL(RouteStaticResources, path, argp)
     else
     {
         HttpResponseStatus(args->context, HTTP_NOT_FOUND);
-        return MatrixErrorCreate(M_NOT_FOUND);
+        return MatrixErrorCreate(M_NOT_FOUND, NULL);
     }
 
 
diff --git a/src/Routes/RouteTokenValid.c b/src/Routes/RouteTokenValid.c
index e17cf43..157eb3b 100644
--- a/src/Routes/RouteTokenValid.c
+++ b/src/Routes/RouteTokenValid.c
@@ -47,14 +47,14 @@ ROUTE_IMPL(RouteTokenValid, path, argp)
     if (HttpRequestMethodGet(args->context) != HTTP_GET)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        return MatrixErrorCreate(M_UNRECOGNIZED);
+        return MatrixErrorCreate(M_UNRECOGNIZED, NULL);
     }
 
     request = JsonDecode(HttpServerStream(args->context));
     if (!request)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        return MatrixErrorCreate(M_NOT_JSON);
+        return MatrixErrorCreate(M_NOT_JSON, NULL);
     }
 
     tokenstr = JsonValueAsString(HashMapGet(request, "token"));
diff --git a/src/Routes/RouteUiaFallback.c b/src/Routes/RouteUiaFallback.c
index eb4c2de..88e477b 100644
--- a/src/Routes/RouteUiaFallback.c
+++ b/src/Routes/RouteUiaFallback.c
@@ -41,7 +41,7 @@ ROUTE_IMPL(RouteUiaFallback, path, argp)
         /* This should never happen */
         Log(LOG_ERR, "Programmer error in RouteUiaFallback()!");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        return MatrixErrorCreate(M_UNKNOWN);
+        return MatrixErrorCreate(M_UNKNOWN, NULL);
     }
 
     if (HttpRequestMethodGet(args->context) == HTTP_POST)
@@ -58,7 +58,7 @@ ROUTE_IMPL(RouteUiaFallback, path, argp)
         {
             Log(LOG_ERR, "UIA fallback failed to lock configuration.");
             HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-            return MatrixErrorCreate(M_UNKNOWN);
+            return MatrixErrorCreate(M_UNKNOWN, NULL);
         }
 
         request = JsonDecode(HttpServerStream(args->context));
@@ -66,7 +66,7 @@ ROUTE_IMPL(RouteUiaFallback, path, argp)
         {
             ConfigUnlock(config);
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            return MatrixErrorCreate(M_NOT_JSON);
+            return MatrixErrorCreate(M_NOT_JSON, NULL);
         }
 
         flow = ArrayCreate();
@@ -80,7 +80,7 @@ ROUTE_IMPL(RouteUiaFallback, path, argp)
         if (uiaResult < 0)
         {
             HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-            response = MatrixErrorCreate(M_UNKNOWN);
+            response = MatrixErrorCreate(M_UNKNOWN, NULL);
         }
         else if (uiaResult)
         {
@@ -94,14 +94,14 @@ ROUTE_IMPL(RouteUiaFallback, path, argp)
     else if (HttpRequestMethodGet(args->context) != HTTP_GET)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        return MatrixErrorCreate(M_UNRECOGNIZED);
+        return MatrixErrorCreate(M_UNRECOGNIZED, NULL);
     }
 
     sessionId = HashMapGet(requestParams, "session");
     if (!sessionId)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        return MatrixErrorCreate(M_MISSING_PARAM);
+        return MatrixErrorCreate(M_MISSING_PARAM, NULL);
     }
 
     HttpResponseHeader(args->context, "Content-Type", "text/html");
diff --git a/src/Routes/RouteUserProfile.c b/src/Routes/RouteUserProfile.c
index 4a741bb..0c99338 100644
--- a/src/Routes/RouteUserProfile.c
+++ b/src/Routes/RouteUserProfile.c
@@ -54,7 +54,7 @@ ROUTE_IMPL(RouteUserProfile, path, argp)
     {
         Log(LOG_ERR, "User profile endpoint failed to lock configuration.");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        return MatrixErrorCreate(M_UNKNOWN);
+        return MatrixErrorCreate(M_UNKNOWN, NULL);
     }
 
     serverName = config->serverName;
@@ -64,14 +64,14 @@ ROUTE_IMPL(RouteUserProfile, path, argp)
     if (!userId)
     {
         HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-        response = MatrixErrorCreate(M_INVALID_PARAM);
+        response = MatrixErrorCreate(M_INVALID_PARAM, NULL);
         goto finish;
     }
     if (strcmp(userId->server, serverName))
     {
         /* TODO: Implement lookup over federation. */
         HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-        response = MatrixErrorCreate(M_FORBIDDEN);
+        response = MatrixErrorCreate(M_FORBIDDEN, NULL);
         goto finish;
     }
 
@@ -83,7 +83,7 @@ ROUTE_IMPL(RouteUserProfile, path, argp)
             if (!user)
             {
                 HttpResponseStatus(args->context, HTTP_NOT_FOUND);
-                response = MatrixErrorCreate(M_NOT_FOUND);
+                response = MatrixErrorCreate(M_NOT_FOUND, NULL);
                 goto finish;
             }
 
@@ -118,7 +118,7 @@ ROUTE_IMPL(RouteUserProfile, path, argp)
                 if (!request)
                 {
                     HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                    response = MatrixErrorCreate(M_NOT_JSON);
+                    response = MatrixErrorCreate(M_NOT_JSON, NULL);
                     goto finish;
                 }
                 response = MatrixGetAccessToken(args->context, &token);
@@ -130,7 +130,7 @@ ROUTE_IMPL(RouteUserProfile, path, argp)
                 if (!user)
                 {
                     HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                    response = MatrixErrorCreate(M_UNKNOWN_TOKEN);
+                    response = MatrixErrorCreate(M_UNKNOWN_TOKEN, NULL);
                     goto finish;
                 }
                 entry = ArrayGet(path, 1);
@@ -149,25 +149,25 @@ ROUTE_IMPL(RouteUserProfile, path, argp)
                     }
                     /* User is not allowed to carry-on the action */
                     HttpResponseStatus(args->context, HTTP_FORBIDDEN);
-                    response = MatrixErrorCreate(M_FORBIDDEN);
+                    response = MatrixErrorCreate(M_FORBIDDEN, NULL);
                     goto finish;
                 }
                 else
                 {
                     HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                    response = MatrixErrorCreate(M_UNRECOGNIZED);
+                    response = MatrixErrorCreate(M_UNRECOGNIZED, NULL);
                     goto finish;
                 }
             }
             else
             {
                 HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-                response = MatrixErrorCreate(M_MISSING_PARAM);
+                response = MatrixErrorCreate(M_MISSING_PARAM, NULL);
                 goto finish;
             }
         default:
             HttpResponseStatus(args->context, HTTP_BAD_REQUEST);
-            response = MatrixErrorCreate(M_UNKNOWN);
+            response = MatrixErrorCreate(M_UNKNOWN, NULL);
             break;
     }
 finish:
diff --git a/src/Routes/RouteWellKnown.c b/src/Routes/RouteWellKnown.c
index b62169f..855bf90 100644
--- a/src/Routes/RouteWellKnown.c
+++ b/src/Routes/RouteWellKnown.c
@@ -41,7 +41,7 @@ ROUTE_IMPL(RouteWellKnown, path, argp)
     {
         Log(LOG_ERR, "Well-known endpoint failed to lock configuration.");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        return MatrixErrorCreate(M_UNKNOWN);
+        return MatrixErrorCreate(M_UNKNOWN, NULL);
     }
 
     if (StrEquals(ArrayGet(path, 0), "client"))
@@ -51,7 +51,7 @@ ROUTE_IMPL(RouteWellKnown, path, argp)
     else
     {
         HttpResponseStatus(args->context, HTTP_NOT_FOUND);
-        response = MatrixErrorCreate(M_NOT_FOUND);
+        response = MatrixErrorCreate(M_NOT_FOUND, NULL);
     }
 
     ConfigUnlock(config);
diff --git a/src/Routes/RouteWhoami.c b/src/Routes/RouteWhoami.c
index 9532a46..66bf7c0 100644
--- a/src/Routes/RouteWhoami.c
+++ b/src/Routes/RouteWhoami.c
@@ -49,7 +49,7 @@ ROUTE_IMPL(RouteWhoami, path, argp)
     {
         Log(LOG_ERR, "Who am I endpoint failed to lock configuration.");
         HttpResponseStatus(args->context, HTTP_INTERNAL_SERVER_ERROR);
-        return MatrixErrorCreate(M_UNKNOWN);
+        return MatrixErrorCreate(M_UNKNOWN, NULL);
     }
 
     (void) path;
@@ -67,7 +67,7 @@ ROUTE_IMPL(RouteWhoami, path, argp)
     if (!user)
     {
         HttpResponseStatus(args->context, HTTP_UNAUTHORIZED);
-        response = MatrixErrorCreate(M_UNKNOWN_TOKEN);
+        response = MatrixErrorCreate(M_UNKNOWN_TOKEN, NULL);
         goto finish;
     }
 
diff --git a/src/Uia.c b/src/Uia.c
index c2bedf0..de6b04e 100644
--- a/src/Uia.c
+++ b/src/Uia.c
@@ -243,7 +243,7 @@ UiaComplete(Array * flows, HttpServerContext * context, Db * db,
     if (JsonValueType(val) != JSON_OBJECT)
     {
         HttpResponseStatus(context, HTTP_BAD_REQUEST);
-        *response = MatrixErrorCreate(M_BAD_JSON);
+        *response = MatrixErrorCreate(M_BAD_JSON, NULL);
         return 0;
     }
 
@@ -253,7 +253,7 @@ UiaComplete(Array * flows, HttpServerContext * context, Db * db,
     if (!val || JsonValueType(val) != JSON_STRING)
     {
         HttpResponseStatus(context, HTTP_BAD_REQUEST);
-        *response = MatrixErrorCreate(M_BAD_JSON);
+        *response = MatrixErrorCreate(M_BAD_JSON, NULL);
         return 0;
     }
 
@@ -312,7 +312,7 @@ UiaComplete(Array * flows, HttpServerContext * context, Db * db,
     if (!val || JsonValueType(val) != JSON_STRING)
     {
         HttpResponseStatus(context, HTTP_BAD_REQUEST);
-        *response = MatrixErrorCreate(M_BAD_JSON);
+        *response = MatrixErrorCreate(M_BAD_JSON, NULL);
         ret = 0;
         goto finish;
     }
diff --git a/src/include/Matrix.h b/src/include/Matrix.h
index 2239906..1d71e83 100644
--- a/src/include/Matrix.h
+++ b/src/include/Matrix.h
@@ -110,9 +110,10 @@ extern void MatrixHttpHandler(HttpServerContext *, void *);
 
 /**
  * A convenience function that constructs an error payload, including
- * the error code and message, given just a MatrixError.
+ * the error code and message, given a MatrixError and an optional 
+ * message.
  */
-extern HashMap * MatrixErrorCreate(MatrixError);
+extern HashMap * MatrixErrorCreate(MatrixError, char *);
 
 /**
  * Read the request headers and parameters, and attempt to obtain an