diff -r 1df7e7b8558e -r 04c319a07c7b mercurial/ui.py
--- a/mercurial/ui.py	Sun Feb 25 22:30:14 2018 -0500
+++ b/mercurial/ui.py	Sun Feb 25 23:08:41 2018 -0500
@@ -366,7 +366,7 @@
         except error.ConfigError as inst:
             if trusted:
                 raise
-            self.warn(_("ignored: %s\n") % str(inst))
+            self.warn(_("ignored: %s\n") % util.forcebytestr(inst))
 
         if self.plain():
             for k in ('debug', 'fallbackencoding', 'quiet', 'slash',