diff --git a/src/calibre/ebooks/metadata/worker.py b/src/calibre/ebooks/metadata/worker.py
index 7dff988679..a7a8177176 100644
--- a/src/calibre/ebooks/metadata/worker.py
+++ b/src/calibre/ebooks/metadata/worker.py
@@ -271,6 +271,8 @@ class SaveWorker(Thread):
                 break
 
         for job in jobs:
+            if not job.result:
+                continue
             for id_, title, ok, tb in job.result:
                 if id_ in ids:
                     self.result_queue.put((id_, title, ok, tb))