CustomEvent for IE8 or other old browsers that do not implement it. `new CustomEvent('type', {bubbles:true, detail:{what:'ever'}})`

CustomEvent.js

try{new CustomEvent('?')}catch(o_O){
  /*!(C) Andrea Giammarchi -- WTFPL License*/
  this.CustomEvent = function(
    eventName,
    defaultInitDict
  ){

    // the infamous substitute
    function CustomEvent(type, eventInitDict) {
      var event = document.createEvent(eventName);
      if (type != null) {
        initCustomEvent.call(
          event,
          type,
          (eventInitDict || (
            // if falsy we can just use defaults
            eventInitDict = defaultInitDict
          )).bubbles,
          eventInitDict.cancelable,
          eventInitDict.detail
        );
      } else {
        // no need to put the expando property otherwise
        // since an event cannot be initialized twice
        // previous case is the most common one anyway
        // but if we end up here ... there it goes
        event.initCustomEvent = initCustomEvent;
      }
      return event;
    }

    // borrowed or attached at runtime
    function initCustomEvent(
      type, bubbles, cancelable, detail
    ) {
      this['init' + eventName](type, bubbles, cancelable, detail);
      'detail' in this || (this.detail = detail);
    }

    // that's it
    return CustomEvent;
  }(
    // is this IE9 or IE10 ?
    // where CustomEvent is there
    // but not usable as construtor ?
    this.CustomEvent ?
      // use the CustomEvent interface in such case
      'CustomEvent' : 'Event',
      // otherwise the common compatible one
    {
      bubbles: false,
      cancelable: false,
      detail: null
    }
  );
}

line 10: var event = document.createEvent(eventName); causes "Object doesn't support property or method 'createEvent'" in IE8.

Is @AJ-Tucker comment true? I can't reproduce it. For me the code works, and works as expected.