diff --git a/src/calibre/ebooks/chm/metadata.py b/src/calibre/ebooks/chm/metadata.py
index d6a1d24024..28e307df95 100644
--- a/src/calibre/ebooks/chm/metadata.py
+++ b/src/calibre/ebooks/chm/metadata.py
@@ -19,6 +19,8 @@ def _clean(s):
 
 def _detag(tag):
     str = u""
+    if tag is None:
+        return str
     for elem in tag:
         if hasattr(elem, "contents"):
             str += _detag(elem)