diff --git a/mercurial/shelve.py b/mercurial/shelve.py
--- a/mercurial/shelve.py
+++ b/mercurial/shelve.py
@@ -699,7 +699,7 @@
             if shfile.exists():
                 shfile.movetobackup()
         cleanupoldbackups(repo)
-def unshelvecontinue(ui, repo, state, opts, basename=None):
+def unshelvecontinue(ui, repo, state, opts, basename):
     """subcommand to continue an in-progress unshelve"""
     # We're finishing off a merge. First parent is our original
     # parent, second is the temporary "fake" commit we're unshelving.