--- a/mercurial/obsutil.py	Mon Jul 24 11:28:59 2017 -0400
+++ b/mercurial/obsutil.py	Mon Jul 24 11:29:51 2017 -0400
@@ -307,7 +307,7 @@
         seenrevs.add(rev)
         if phase(repo, rev) == public:
             continue
-        if set(succsmarkers(node)).issubset(addedmarkers):
+        if set(succsmarkers(node) or []).issubset(addedmarkers):
             obsoleted.add(rev)
     return obsoleted