--- a/mercurial/obsolete.py	Sat Sep 27 13:18:10 2014 -0500
+++ b/mercurial/obsolete.py	Tue Sep 23 12:21:38 2014 -0700
@@ -852,7 +852,7 @@
     Such access may compute the set and cache it for future use"""
     repo = repo.unfiltered()
     if not repo.obsstore:
-        return ()
+        return frozenset()
     if name not in repo.obsstore.caches:
         repo.obsstore.caches[name] = cachefuncs[name](repo)
     return repo.obsstore.caches[name]