diff --git a/src/calibre/ebooks/conversion/preprocess.py b/src/calibre/ebooks/conversion/preprocess.py
index e3e6983742..f6ef3d295e 100644
--- a/src/calibre/ebooks/conversion/preprocess.py
+++ b/src/calibre/ebooks/conversion/preprocess.py
@@ -452,7 +452,7 @@ def book_designer_rules():
         (re.compile('<span[^><]*?id=subtitle[^><]*?>(.*?)</span>', re.IGNORECASE|re.DOTALL),
         lambda match : '<h3 class="subtitle">%s</h3>'%(match.group(1),)),
     ]
-    return None
+    return ans
 
 
 class HTMLPreProcessor(object):