diff -r 6e2c8f7f894e -r d1bc6cf2be69 mercurial/exchange.py
--- a/mercurial/exchange.py	Sat Sep 29 20:57:49 2018 -0400
+++ b/mercurial/exchange.py	Sat Sep 29 23:52:00 2018 -0400
@@ -522,7 +522,8 @@
         # source repo cannot be locked.
         # We do not abort the push, but just disable the local phase
         # synchronisation.
-        msg = 'cannot lock source repository: %s\n' % err
+        msg = ('cannot lock source repository: %s\n'
+               % stringutil.forcebytestr(err))
         pushop.ui.debug(msg)
 
     with wlock or util.nullcontextmanager(), \