diff --git a/src/Matrix.c b/src/Matrix.c
index c51b04b..9f87928 100644
--- a/src/Matrix.c
+++ b/src/Matrix.c
@@ -432,7 +432,7 @@ MatrixAuthenticate(HttpServerContext * context, Db * db)
         token += 8;
 
         /* Seek past any spaces between "Bearer" and the token */
-        while (*token && isspace(*token))
+        while (*token && isspace((unsigned char) *token))
         {
             token++;
         }