Fixing python error when connecting with an old cookie

here's the callstack:

  File "/app/.heroku/src/flask-login/flask_login.py", line 337, in _load_user 
May 25 14:33:33 mightyspring app/web.1:      self._load_from_cookie(request.cookies[cookie_name]) 
May 25 14:33:33 mightyspring app/web.1:    File "/app/.heroku/src/flask-login/flask_login.py", line 381, in _load_from_cookie 
May 25 14:33:33 mightyspring app/web.1:      if user_id is not None: 
May 25 14:33:33 mightyspring app/web.1:  UnboundLocalError: local variable 'user_id' referenced before assignment