diff --git a/glx/glxcmds.c b/glx/glxcmds.c
index ec3bbe6a3..34829dd7d 100644
--- a/glx/glxcmds.c
+++ b/glx/glxcmds.c
@@ -57,7 +57,7 @@ validGlxScreen(ClientPtr client, int screen, __GLXscreen **pGlxScreen, int *err)
     /*
     ** Check if screen exists.
     */
-    if (screen >= screenInfo.numScreens) {
+    if (screen < 0 || screen >= screenInfo.numScreens) {
 	client->errorValue = screen;
 	*err = BadValue;
 	return FALSE;