--- a/mercurial/changegroup.py	Sun Mar 19 01:47:56 2017 -0400
+++ b/mercurial/changegroup.py	Fri Mar 24 08:37:26 2017 -0700
@@ -294,7 +294,7 @@
                 # will not see an inconsistent view
                 cl = repo.changelog
                 cl.delayupdate(tr)
-                oldheads = cl.heads()
+                oldheads = set(cl.heads())
 
                 trp = weakref.proxy(tr)
                 # pull off the changeset group