diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -366,7 +366,9 @@
         obsrevs = obsolete.getrevs(repo, b'obsolete')
         if not cl.filteredrevs and not obsrevs:
             return None
-        key = (maxrev, hash(cl.filteredrevs), hash(obsrevs))
+        # TODO: obsrevs should be a frozenset, but right now obsolete.getrevs()
+        # may return a set, which is not a hashable type.
+        key = (maxrev, hash(cl.filteredrevs), hash(frozenset(obsrevs)))
     else:
         if not cl.filteredrevs:
             return None