diff --git a/common/rfb/SSecurityTLS.cxx b/common/rfb/SSecurityTLS.cxx
index f54158e..52fc9cb 100644
--- a/common/rfb/SSecurityTLS.cxx
+++ b/common/rfb/SSecurityTLS.cxx
@@ -23,7 +23,9 @@
 #include <config.h>
 #endif
 
-#ifdef HAVE_GNUTLS
+#ifndef HAVE_GNUTLS
+#error "This source should not be compiled without HAVE_GNUTLS defined"
+#endif
 
 #include <rfb/SSecurityTLS.h>
 #include <rfb/Exception.h>
@@ -79,4 +81,3 @@
 
 }
 
-#endif /* HAVE_GNUTLS */