Query to undefined location because of value-binding to object?

I've got a controller like this:

import Ember from 'ember';

export default Ember.ArrayController.extend({
  currentTrack: {'http://ift.tt/1E6GyHF'}
})

and a template-file like this:

<div class="song">
  <img {{bind-attr src=currentTrack.artwork_url}} />
</div>

For some reason this causes Ember to send a HTTP-request to http://localhost:4200/undefined . It doesn't happen when currentTrack is just a string.

How can I prevent that but still us currentTrack as an object (it normally has more attributes. I just left them out for the sake of simplicity)?