diff -r 46c97973ee46 -r af0a19d8812b mercurial/wireprotoserver.py
--- a/mercurial/wireprotoserver.py	Sat Feb 17 01:09:56 2018 -0500
+++ b/mercurial/wireprotoserver.py	Sat Feb 17 01:11:48 2018 -0500
@@ -339,7 +339,7 @@
 
     # TODO This response body assumes the failed command was
     # "unbundle." That assumption is not always valid.
-    req.respond(e, HGTYPE, body='0\n%s\n' % e)
+    req.respond(e, HGTYPE, body='0\n%s\n' % pycompat.bytestr(e))
 
     return ''