Traceur runtime: Super expression must either be null or a function, not undefined

Learning ES6 and have run in to the following error straight off `Super expression must either be null or a function, not undefined.` Really unsure where my problem is, if anyone could help that would be great. main.js ``` 'use strict' import Backbone from 'exoskeleton'; import App from './views/App'; var onDOMReady = () => { console.log('inside dom ready'); window.app = new App(); } if(document.readyState === 'complete' || document.readyState === 'interactive' || document.readyState === 'loaded' ) { onDOMReady(); } else { document.addEventListener('DOMContentLoaded', onDOMReady); } ``` App.js ``` 'use strict' import Backbone from 'exoskeleton'; class App extends Backbone.View { initialize () { console.log('App: Init'); } render () { console.log('App: Render'); } } export default App; ```