diff -r 515014d72aa2 -r c887bab2dccf mercurial/localrepo.py
--- a/mercurial/localrepo.py	Fri Jun 11 23:51:27 2021 -0400
+++ b/mercurial/localrepo.py	Fri Jun 11 23:57:14 2021 -0400
@@ -2759,7 +2759,7 @@
                 "`full` argument for `repo.updatecaches` is deprecated\n"
                 "(use `caches=repository.CACHE_ALL` instead)"
             )
-            self.ui.deprecwarn(msg, "5.9")
+            self.ui.deprecwarn(msg, b"5.9")
             caches = repository.CACHES_ALL
             if full == b"post-clone":
                 caches = repository.CACHES_POST_CLONE