Computed not firing on hash

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  hash: {
    firstItem: 'abc',
  },
  getFirstItem: Ember.computed('hash', function() {
    return this.get('hash.firstItem');
  }),
  actions: {
    changeHash() {
      this.set('hash', {firstItem: 'xyz'});
    }
  }
});

application.template.hbs

First item is: {{getFirstItem}} (computed)<br />
First item is: {{hash.firstItem}} (actual)
<br />

Update: {{input value=hash.firstItem}}

<button {{action 'changeHash'}} type='button'>ChangeHash</button>

twiddle.json

{
  "version": "0.4.14",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.1.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.1.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.1.0/ember-template-compiler.js"
  }
}