diff --git a/hw/display/qxl.c b/hw/display/qxl.c
index 0b21626aad..6772849dec 100644
--- a/hw/display/qxl.c
+++ b/hw/display/qxl.c
@@ -1384,6 +1384,7 @@ static int qxl_add_memslot(PCIQXLDevice *d, uint32_t slot_id, uint64_t delta,
         qxl_set_guest_bug(d, "%s: pci_region = %d", __func__, pci_region);
         return 1;
     }
+    assert(guest_end - pci_start <= memory_region_size(mr));
 
     virt_start = (intptr_t)memory_region_get_ram_ptr(mr);
     memslot.slot_id = slot_id;