--- a/mercurial/templatefuncs.py	Wed Dec 12 22:19:57 2018 +0900
+++ b/mercurial/templatefuncs.py	Wed Dec 12 22:45:02 2018 +0900
@@ -609,7 +609,7 @@
 
     match = patre.search(src)
     if not match:
-        return
+        return templateutil.mappingnone()
 
     lm = {b'0': match.group(0)}
     lm.update((b'%d' % i, v) for i, v in enumerate(match.groups(), 1))