--- xorg-server-1.7.7.orig/hw/dmx/glxProxy/glxcmdsswap.c
+++ xorg-server-1.7.7/hw/dmx/glxProxy/glxcmdsswap.c
@@ -251,7 +251,7 @@
 
     __GLX_SWAP_INT(&req->screen);
     screen = req->screen;
-    if (screen > screenInfo.numScreens) {
+    if (screen >= screenInfo.numScreens) {
 	/* The client library must send a valid screen number. */
 	client->errorValue = screen;
 	return BadValue;