diff --git a/Modules/CMakeTestGNU.c b/Modules/CMakeTestGNU.c
index 1cd56244b..63fd2b240 100644
--- a/Modules/CMakeTestGNU.c
+++ b/Modules/CMakeTestGNU.c
@@ -1,3 +1,3 @@
 #ifdef __GNUC__
-THIS_IS_GNU
+void THIS_IS_GNU();
 #endif