Built-ins intents failed on dev portal after adding google analytics

I am building a custom skill. Before adding google analytics, the 3 built-ins & 2 customs intents were all working properly, both when tested from AWS console and the service simulator. Now, I get 'The response is invalid' from the service simulator, ONLY for the built-ins intents (the 2 customs intents give a correct lambda response). Here is what have changed in my code since the last working version:

Adding:

var express = require('express');
var request = require('request');
var app = express();
var GA_TRACKING_ID = 'UA_TRACKING_ID';

function trackEvent(category, action, label, value, cb) {
  console.log('1');
  var data = {v: '1', tid: GA_TRACKING_ID, cid: '555', t: 'event', ec: category, ea: action, el: label, ev: value,};
  request.post(
    'http://www.google-analytics.com/collect', {
      form: data
    },
    function(err, response) {
      console.log('2');
      if (err) { console.log('3'); return cb(err); }
      if (response.statusCode !== 200) {
        console.log('4');
        return cb(new Error('Tracking failed'));
      }
      console.log('5');
      cb();
    }
  );
}

And calling the trackEvent function, similarly in all intents:

Built-in intent example: (=> testing this intent logs out '1' in the CloudWatch, and then execution stops)

'AMAZON.CancelIntent': function (intent, session, response) {
      var self = this;
      trackEvent('Intent', 'AMAZON.CancelIntent', 'stop', '100', function(err) {
        if (err) { return next(err); }
        self.response.speak(STOP_MESSAGE);
        self.emit(':responseReady');
      })

Example in a custom intent: (=> testing this intent will output the correct speechOutput)

'DressingTodayIntent': function(intent, session, response) {
      var speechOutput;
      var self = this;
      var cb = getJSON('...', function(err, forecast) {
          if (err) {
            console.log('Error occurred while trying to retrieve data', err);
          } else {
            speechOutput = getDressingAdvice(forecast, true);
          }
          self.response.cardRenderer("Your advice for today:", speechOutput);
          self.response.speak(speechOutput);
          self.emit(':responseReady');
        });
      trackEvent('Intent', 'DressingTodayIntent', 'DressingToday', '100', cb);
    }

I insist, 'AMAZON.CancelIntent', 'AMAZON.HelpIntent', 'AMAZON.StopIntent' (& 'LaunchRequest') are not working only since I added GA. What did I do wrong?

Last reference, the working intent code, before adding GA:

'AMAZON.CancelIntent': function () {
      this.response.speak(STOP_MESSAGE);
      this.emit(':responseReady');
    },