diff --git a/lib/urldata.h b/lib/urldata.h
index e918cd61d026485f0c119380347f5dd50ce60e29..e9e4fb87265502558035be86f522b8ebfc9327b1 100644
--- a/lib/urldata.h
+++ b/lib/urldata.h
@@ -73,6 +73,9 @@
 #include "ssl.h"
 #include "err.h"
 #endif /* USE_OPENSSL */
+#ifdef USE_GNUTLS
+#error Configuration error; cannot use GnuTLS *and* OpenSSL.
+#endif
 #endif /* USE_SSLEAY */
 
 #ifdef USE_GNUTLS