diff -r 057d31ceace3 -r 38a3767975a7 mercurial/extensions.py
--- a/mercurial/extensions.py	Sat Aug 12 04:47:40 2017 +0530
+++ b/mercurial/extensions.py	Tue Jul 25 22:49:43 2017 -0400
@@ -384,6 +384,7 @@
 
     These can't be wrapped using the normal wrapfunction.
     """
+    propname = pycompat.sysstr(propname)
     assert callable(wrapper)
     for currcls in cls.__mro__:
         if propname in currcls.__dict__: