diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index 98c62f2..e5c6e7c 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -518,6 +518,9 @@ VirtIODevice *virtio_blk_init(DeviceState *dev, BlockConf *conf)
     bdrv_guess_geometry(s->bs, &cylinders, &heads, &secs);
 
     strncpy(s->sn, drive_get_serial(s->bs), sizeof (s->sn));
+    if (strlen(s->sn) == 0) {
+        snprintf(s->sn, sizeof(s->sn), "QM%05d", conf->dinfo->unit);
+    }
 
     s->vq = virtio_add_queue(&s->vdev, 128, virtio_blk_handle_output);