diff -r 4237398c67c0 -r 593ad8df9dd2 mercurial/bundle2.py
--- a/mercurial/bundle2.py	Fri Jun 30 03:44:08 2017 +0200
+++ b/mercurial/bundle2.py	Fri Jun 30 03:44:09 2017 +0200
@@ -1343,7 +1343,7 @@
         caps['obsmarkers'] = supportedformat
     if allowpushback:
         caps['pushback'] = ()
-    cpmode = repo.ui.config('server', 'concurrent-push-mode', 'strict')
+    cpmode = repo.ui.config('server', 'concurrent-push-mode')
     if cpmode == 'check-related':
         caps['checkheads'] = ('related',)
     return caps