diff -r 6b52cffd8d0a -r e2f7b2695ba1 mercurial/upgrade_utils/engine.py
--- a/mercurial/upgrade_utils/engine.py	Tue Mar 02 00:05:22 2021 +0100
+++ b/mercurial/upgrade_utils/engine.py	Thu Mar 18 18:24:59 2021 -0400
@@ -581,6 +581,7 @@
             # reference to its new location. So clean it up manually. Alternatively, we
             # could update srcrepo.svfs and other variables to point to the new
             # location. This is simpler.
+            assert backupvfs is not None  # help pytype
             backupvfs.unlink(b'store/lock')
 
     return backuppath