title: what is uri / server error
body: <p>I have a uri / server error and I don't understand what it means.</p>
  
  <p>my mapping looks like this:</p>
  
  <p>app = webapp2.WSGIApplication([('/',  MainPage), ('/fizzbuzz',  FizzBuzzHandler),
                                 ],
                                debug=True)</p>
  
  <p>and the error log on app engine is <a href="https://i.stack.imgur.com/n8NBY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/n8NBY.png" alt="the server error log on cloud console"></a></p>