title: Error with Facebook messenger bot code
body: <p>I'm trying to follow the quickstart Facebook Messenger Bot code (found here: <a href="https://developers.facebook.com/docs/messenger-platform/quickstart" rel="nofollow">https://developers.facebook.com/docs/messenger-platform/quickstart</a>)</p>
  
  <p>And I'm getting this error:</p>
  
  <p>{"error":{"message":"(#100) param recipient must be non-empty.","type":"OAuthException","code":100,"fbtrace_id":"ALtKsX4nJtX"}}</p>
  
  <p>Any ideas? Seems like I'm returning the right response but for some reason Facebook isn't accepting it? Thanks</p>
  
  <p>Code as follows (python/appengine):</p>
  
  <pre><code>import webapp2
  import json
  import urllib
  from google.appengine.api import urlfetch
  
  class MainHandler(webapp2.RequestHandler):
      print "hello"
  
  FB_MESSENGER_ACCESS_TOKEN = ""
  
  def respond_FB(sender_id, text):
      json_data = {
          "recipient": {"id": sender_id},
          "message": {"text": text + " to you!"}
      }
      url = "https://graph.facebook.com/v2.6/me/messages?access_token="+FB_MESSENGER_ACCESS_TOKEN
      print("json_data", json_data)
      data = urllib.urlencode(json_data)
      response = urlfetch.fetch(url = url, payload = data, method = urlfetch.POST, headers = {"Content-Type": "application/json"})
      print(response.content)
  
  class FacebookMessenger(webapp2.RequestHandler):
      def get(self):
          if (self.request.get("hub.verify_token") == 'tomstoken'):
              return self.response.write(self.request.get("hub.challenge"))
          return self.response.write('Error, wrong validation token')
  
  
      def post(self):
          body = self.request.body
          messaging_events = json.loads(body.decode("utf-8"))
          sender_id = messaging_events["entry"][0]["messaging"][0]["sender"]["id"]
          message = messaging_events["entry"][0]["messaging"][0]["message"]["text"]
          respond_FB(sender_id, message)
          return self.response.write('Received.')
  
  app = webapp2.WSGIApplication([
      ('/', MainHandler),
      ('/webhook*', FacebookMessenger)
  ], debug=True)
  </code></pre>