diff --git a/youtube_dl/extractor/common.py b/youtube_dl/extractor/common.py
index eaae5e484..c51a3a07d 100644
--- a/youtube_dl/extractor/common.py
+++ b/youtube_dl/extractor/common.py
@@ -2340,6 +2340,8 @@ class InfoExtractor(object):
         if res is False:
             return []
         ism_doc, urlh = res
+        if ism_doc is None:
+            return []
 
         return self._parse_ism_formats(ism_doc, urlh.geturl(), ism_id)