diff --git a/dix/inpututils.c b/dix/inpututils.c
index 79b6ab7..c140cdd 100644
--- a/dix/inpututils.c
+++ b/dix/inpututils.c
@@ -55,7 +55,7 @@ check_butmap_change(DeviceIntPtr dev, CARD8 *map, int len, CARD32 *errval_out,
         if (dev->button->map[i + 1] != map[i] && dev->button->down[i + 1])
             return MappingBusy;
 
-        if (map[i] < 1 || map[i] > 255) {
+        if (map[i] && (map[i] < 1 || map[i] > 255)) {
             if (errval_out)
                 *errval_out = map[i];
             return -1;