diff --git a/kernel/trunk/core/memory.inc b/kernel/trunk/core/memory.inc
index 6a17c174d7..e29154b1ec 100644
--- a/kernel/trunk/core/memory.inc
+++ b/kernel/trunk/core/memory.inc
@@ -1054,6 +1054,8 @@ f68:
            mov [esp+36], eax
            ret
 .25:
+           cmp ebx,32
+           jae .fail
            mov  eax, [current_slot]
            btr  [eax+APPDATA.except_mask],ebx
            setc byte[esp+36]