diff --git a/dix/inpututils.c b/dix/inpututils.c
index 2226d5562..c6300880b 100644
--- a/dix/inpututils.c
+++ b/dix/inpututils.c
@@ -235,6 +235,9 @@ static int build_modmap_from_modkeymap(CARD8 *modmap, KeyCode *modkeymap,
         if (!modkeymap[i])
             continue;
 
+        if (modkeymap[i] >= MAP_LENGTH)
+            return BadValue;
+
         if (modmap[modkeymap[i]])
             return BadValue;