diff -r 7cf9a9e0cf89 -r 6c55e37ba5f2 mercurial/templater.py
--- a/mercurial/templater.py	Wed Mar 11 15:22:34 2015 -0700
+++ b/mercurial/templater.py	Tue Feb 24 00:04:55 2015 +0900
@@ -162,8 +162,13 @@
 
 def runfilter(context, mapping, data):
     func, data, filt = data
+    # func() may return string, generator of strings or arbitrary object such
+    # as date tuple, but filter does not want generator.
+    thing = func(context, mapping, data)
+    if isinstance(thing, types.GeneratorType):
+        thing = stringify(thing)
     try:
-        return filt(func(context, mapping, data))
+        return filt(thing)
     except (ValueError, AttributeError, TypeError):
         if isinstance(data, tuple):
             dt = data[1]