diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -299,9 +299,7 @@
                     msg
                     % (
                         _branchcachedesc(repo),
-                        pycompat.bytestr(
-                            inst
-                        ),  # pytype: disable=wrong-arg-types
+                        stringutil.forcebytestr(inst),
                     )
                 )
             bcache = None