diff --git a/hw/display/qxl.c b/hw/display/qxl.c
index 67efa3c3ef..6b38e55a21 100644
--- a/hw/display/qxl.c
+++ b/hw/display/qxl.c
@@ -1613,7 +1613,10 @@ static void qxl_set_mode(PCIQXLDevice *d, unsigned int modenr, int loadvm)
     }
 
     d->guest_slots[0].slot = slot;
-    assert(qxl_add_memslot(d, 0, devmem, QXL_SYNC) == 0);
+    if (qxl_add_memslot(d, 0, devmem, QXL_SYNC) != 0) {
+        qxl_set_guest_bug(d, "device isn't initialized yet");
+        return;
+    }
 
     d->guest_primary.surface = surface;
     qxl_create_guest_primary(d, 0, QXL_SYNC);