diff --git a/block/snapshot.c b/block/snapshot.c
index 5540655..ad2bf6e 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -311,7 +311,10 @@
          * respective option (with the qdict_put_str() call above).
          * Assert that .bdrv_open() has attached the right BDS as primary child.
          */
+        bdrv_graph_rdlock_main_loop();
         assert(bdrv_primary_bs(bs) == fallback_bs);
+        bdrv_graph_rdunlock_main_loop();
+
         bdrv_unref(fallback_bs);
         return ret;
     }