diff --git a/mercurial/extensions.py b/mercurial/extensions.py
--- a/mercurial/extensions.py
+++ b/mercurial/extensions.py
@@ -549,6 +549,8 @@
       extensions.wrapcommand(commands.table, 'bookmarks', exbookmarks,
                              synopsis, docstring)
     '''
+    assert command == stringutil.forcebytestr(command), \
+        "`command` should be bytes(python3)/str(python2)"
     assert callable(wrapper)
     aliases, entry = cmdutil.findcmd(command, table)
     for alias, e in pycompat.iteritems(table):