diff --git a/block.c b/block.c
index 6dc0d44..271f109 100644
--- a/block.c
+++ b/block.c
@@ -878,6 +878,8 @@
         bs->opaque = NULL;
         bs->drv = NULL;
         bs->copy_on_read = 0;
+        bs->backing_file[0] = '\0';
+        bs->backing_format[0] = '\0';
 
         if (bs->file != NULL) {
             bdrv_close(bs->file);