diff -r 640a0760e666 -r 634997248c97 mercurial/unionrepo.py
--- a/mercurial/unionrepo.py	Fri Jun 30 03:28:28 2017 +0200
+++ b/mercurial/unionrepo.py	Fri Jun 30 03:31:26 2017 +0200
@@ -232,7 +232,7 @@
 def instance(ui, path, create):
     if create:
         raise error.Abort(_('cannot create new union repository'))
-    parentpath = ui.config("bundle", "mainreporoot", "")
+    parentpath = ui.config("bundle", "mainreporoot")
     if not parentpath:
         # try to find the correct path to the working directory repo
         parentpath = cmdutil.findrepo(pycompat.getcwd())