es6 extend parent class undefined

问题:

I have 2 very simple classes one is a parent class Calendar the other the child class MonthCalendar:

calendar.js:

import MonthCalendar from './month-calendar';

export default class Calendar {
  constructor() {
    this.date = new Date();
  }

  displayMonthCalendar(){
    var mcal = new MonthCalendar();
    mcal.generate();
  }
}

var cal = new Calendar();
cal.displayMonthCalendar();

month-calendar.js:

import Calendar from './calendar';

export default class MonthCalendar extends Calendar {
  constructor() {
    super()
  }

  generate() {
    alert('working');
  }
}

When I try running this I keep getting the error:

Uncaught TypeError: Super expression must either be null or a function, not undefined

It's telling me Calendar is undefined; however if I don't extend Calendar and use it inside the generate() method it is not undefined:

month-calendar.js:

import Calendar from './calendar';

export default class MonthCalendar {
  generate() {
    console.log(new Calendar());
  }
}

That throws no error.