--- a/mercurial/upgrade.py	Tue Dec 14 20:36:36 2021 +0100
+++ b/mercurial/upgrade.py	Sun Dec 12 20:01:05 2021 -0500
@@ -42,7 +42,7 @@
 ):
     """Upgrade a repository in place."""
     if optimize is None:
-        optimize = {}
+        optimize = set()
     repo = repo.unfiltered()
 
     specified_revlogs = {}