diff --git a/mercurial/tags.py b/mercurial/tags.py
--- a/mercurial/tags.py
+++ b/mercurial/tags.py
@@ -548,7 +548,7 @@
 
 def _tag(repo, names, node, message, local, user, date, extra=None,
          editor=False):
-    if isinstance(names, str):
+    if isinstance(names, bytes):
         names = (names,)
 
     branches = repo.branchmap()