diff --git a/src/Sha/Sha1.c b/src/Sha/Sha1.c
index dc41922..c4f9db8 100644
--- a/src/Sha/Sha1.c
+++ b/src/Sha/Sha1.c
@@ -29,7 +29,7 @@
 #include <limits.h>
 
 /* TODO: Verify LibreSSL support later */
-#if TLS_IMPL == TLS_OPENSSL
+#if defined(TLS_IMPL) && (TLS_IMPL == TLS_OPENSSL)
 
 #include <openssl/sha.h>
     
diff --git a/src/Sha/Sha256.c b/src/Sha/Sha256.c
index 7f25dae..0b44b7e 100644
--- a/src/Sha/Sha256.c
+++ b/src/Sha/Sha256.c
@@ -31,7 +31,7 @@
 
 
 /* TODO: Verify LibreSSL support later */
-#if TLS_IMPL == TLS_OPENSSL
+#if defined(TLS_IMPL) && (TLS_IMPL == TLS_OPENSSL)
 
 #include <openssl/sha.h>
 
diff --git a/src/Tls/TlsLibreSSL.c b/src/Tls/TlsLibreSSL.c
index dc09b11..3f805ae 100644
--- a/src/Tls/TlsLibreSSL.c
+++ b/src/Tls/TlsLibreSSL.c
@@ -23,7 +23,7 @@
  */
 #include <Tls.h>
 
-#if TLS_IMPL == TLS_LIBRESSL
+#if defined(TLS_IMPL) && (TLS_IMPL == TLS_LIBRESSL)
 
 #include <Memory.h>
 #include <Log.h>
diff --git a/src/Tls/TlsOpenSSL.c b/src/Tls/TlsOpenSSL.c
index cf07ce0..591c6f4 100644
--- a/src/Tls/TlsOpenSSL.c
+++ b/src/Tls/TlsOpenSSL.c
@@ -23,7 +23,7 @@
  */
 #include <Tls.h>
 
-#if TLS_IMPL == TLS_OPENSSL
+#if defined(TLS_IMPL) && (TLS_IMPL == TLS_OPENSSL)
 
 #include <Memory.h>
 #include <Log.h>