I have tried to use twitter follow button and i getting empty event data. Here is my code for the js part. Can i know what is the problem of not getting any data from the event?

window.twttr = (function (d, s, id) {
  var js,
    fjs = d.getElementsByTagName(s)[0],
    t = window.twttr || {};
  if (d.getElementById(id)) return t;
  js = d.createElement(s);
  js.id = id;
  js.src = 'https://platform.twitter.com/widgets.js';
  fjs.parentNode.insertBefore(js, fjs);

  t._e = [];
  t.ready = function (f) {
    t._e.push(f);
  };

  return t;
})(document, 'script', 'twitter-wjs');

twttr.ready(function (twttr) {
  twttr.events.bind('follow', function (event) {
    console.log(event);
    var followedUserId = event.data.user_id;
    var followedScreenName = event.data.screen_name;
  });
});