diff -r 7ce9dea3a14a -r 089fc0db0954 mercurial/unionrepo.py
--- a/mercurial/unionrepo.py	Tue Sep 11 13:46:59 2018 -0700
+++ b/mercurial/unionrepo.py	Tue Sep 11 17:11:32 2018 -0700
@@ -233,7 +233,7 @@
     def getcwd(self):
         return pycompat.getcwd() # always outside the repo
 
-def instance(ui, path, create, intents=None):
+def instance(ui, path, create, intents=None, createopts=None):
     if create:
         raise error.Abort(_('cannot create new union repository'))
     parentpath = ui.config("bundle", "mainreporoot")