diff --git a/block/mirror.c b/block/mirror.c
index 9347533..a62ad86 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -305,7 +305,7 @@ static void coroutine_fn mirror_run(void *opaque)
     }
 
     s->common.len = bdrv_getlength(bs);
-    if (s->common.len < 0) {
+    if (s->common.len <= 0) {
         block_job_completed(&s->common, s->common.len);
         return;
     }