diff --git a/CMakeLists.txt b/CMakeLists.txt
index e70522e35..fa7521d3b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -719,6 +719,9 @@ endif()
 
 CHECK_C_SOURCE_COMPILES(
 	"#include <strings.h>
+	#if __GNUC__
+	#pragma GCC diagnostic error \"-Wimplicit-function-declaration\"
+	#endif
 	int main(int argc, char **argv) { char buf[100]; bzero(buf, 100); return 0; }
 	" LWS_HAVE_BZERO)
 CHECK_C_SOURCE_COMPILES(