diff --git a/hw/qxl.c b/hw/qxl.c
index ac819271b2..bdd36f9d06 100644
--- a/hw/qxl.c
+++ b/hw/qxl.c
@@ -1020,7 +1020,7 @@ void *qxl_phys2virt(PCIQXLDevice *qxl, QXLPHYSICAL pqxl, int group_id)
     case MEMSLOT_GROUP_HOST:
         return (void*)offset;
     case MEMSLOT_GROUP_GUEST:
-        PANIC_ON(slot > NUM_MEMSLOTS);
+        PANIC_ON(slot >= NUM_MEMSLOTS);
         PANIC_ON(!qxl->guest_slots[slot].active);
         PANIC_ON(offset < qxl->guest_slots[slot].delta);
         offset -= qxl->guest_slots[slot].delta;