diff -r 5a1b41268b7c -r 0a2fd3bfc704 mercurial/ui.py
--- a/mercurial/ui.py	Wed Aug 23 01:23:16 2017 -0400
+++ b/mercurial/ui.py	Sun Sep 03 17:47:21 2017 +0900
@@ -595,7 +595,7 @@
             return convert(v)
         except (ValueError, error.ParseError):
             if desc is None:
-                desc = convert.__name__
+                desc = pycompat.sysbytes(convert.__name__)
             raise error.ConfigError(_("%s.%s is not a valid %s ('%s')")
                                     % (section, name, desc, v))