diff --git a/config/ymake-filter.c b/config/ymake-filter.c
index 3f851c781d..efe7d214cc 100644
--- a/config/ymake-filter.c
+++ b/config/ymake-filter.c
@@ -47,7 +47,7 @@ int main()
 {
 	char	*line, *getcppline();
 	int	len, lastlen;
-	int	isacppcomment();
+	int	isacppcomment(char *);
 
 	lastlen = 0;