Serializer error

adapters.application.js

import DS from 'ember-data';

export default DS.RESTAdapter.extend({
  host: 'https://api.github.com'
});

application.controller.js

import Ember from 'ember';
import DS from 'ember-data';

const nullUser = {name: 'Not Found'};

export default Ember.Controller.extend({
  username: 'octocat',
  
  appName: 'Handling serializer errors',
  
  user: Ember.computed('username', function() {
    const promise = this.store
      .find('user', this.get('username'))
      .catch(() => nullUser);
    
    return DS.PromiseObject.create({promise})
  }),
});

application.template.hbs

<h1>{{appName}}</h1>

<p>The fetched user is <strong>{{user.name}}</strong></p>

models.user.js

import DS from 'ember-data';

export default DS.Model.extend({
  name: DS.attr('string'),
});

serializers.application.js

import DS from 'ember-data';

export default DS.JSONSerializer.extend({
  normalize(typeClass, hash) {
    //throw new Error('foo');
    //return undefined;
    return hash;
  }
});

twiddle.json

{
  "version": "0.6.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "1.13.13",
    "ember-data": "1.13.15",
    "ember-template-compiler": "1.13.13"
  }
}