diff --git a/m4/curl-functions.m4 b/m4/curl-functions.m4
index 1c96fb3bf0..c343b93b44 100644
--- a/m4/curl-functions.m4
+++ b/m4/curl-functions.m4
@@ -273,7 +273,7 @@ curl_includes_stdlib="\
 #ifdef HAVE_SYS_TYPES_H
 #  include <sys/types.h>
 #endif
-include <stdlib.h>
+#include <stdlib.h>
 /* includes end */"
   AC_CHECK_HEADERS(
     sys/types.h,