diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -359,8 +359,11 @@
         self.filteredhash = scmutil.filteredhash(repo, self.tiprev)
 
         duration = util.timer() - starttime
+        filtername = repo.filtername
+        if filtername is None:
+            filtername = 'None'
         repo.ui.log('branchcache', 'updated %s branch cache in %.4f seconds\n',
-                    repo.filtername, duration)
+                    repo.filtername or b'None', duration)
 
         self.write(repo)