diff --git a/block.c b/block.c
index 6b29285..d7cb7d4 100644
--- a/block.c
+++ b/block.c
@@ -5381,6 +5381,9 @@
  * deleted. */
 void bdrv_unref(BlockDriverState *bs)
 {
+    if (!bs) {
+        return;
+    }
     assert(bs->refcnt > 0);
     if (--bs->refcnt == 0) {
         bdrv_delete(bs);