diff -Nurp patched/hw/xfree86/common/xisb.c working/hw/xfree86/common/xisb.c
--- patched/hw/xfree86/common/xisb.c	2009-02-02 21:53:58.000000000 +0100
+++ working/hw/xfree86/common/xisb.c	2009-02-02 21:55:09.000000000 +0100
@@ -98,6 +98,9 @@ XisbRead (XISBuffer *b)
 {
 	int ret;
 
+        if (b == NULL)
+            return -2;
+
 	if (b->current >= b->end)
 	{
 		if (b->block_duration >= 0)