diff --git a/proto/x11proto/Xfuncproto.h.in b/proto/x11proto/Xfuncproto.h.in
index 0d7b8b720..dc913face 100644
--- a/proto/x11proto/Xfuncproto.h.in
+++ b/proto/x11proto/Xfuncproto.h.in
@@ -128,10 +128,12 @@ in this Software without prior written authorization from The Open Group.
 # define _X_ATTRIBUTE_PRINTF(x,y)
 #endif
 
-/* requires xproto >= 7.0.22 */
+/* requires xproto >= 7.0.22 - since this uses either gcc or C99 variable
+   argument macros, must be only used inside #ifdef _X_NONNULL guards, as
+   many legacy X clients are compiled in C89 mode still. */
 #if defined(__GNUC__) &&  ((__GNUC__ * 100 + __GNUC_MINOR__) >= 303)
 #define _X_NONNULL(args...)  __attribute__((nonnull(args)))
-#else
+#elif defined(__STDC_VERSION__) && (__STDC_VERSION__ - 0 >= 199901L) /* C99 */
 #define _X_NONNULL(...)  /* */
 #endif