Issue with helper

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  counter: 0,
  
  myArray: Ember.A([1,2,3,4,5]),
  
  actions: {
  	remove() {
      console.log('Removing: ', this.myArray);
    	this.get('myArray').popObject();
    },
    
    add() {
      console.log('Adding: ', this.myArray);
    	this.get('myArray').pushObject(this.counter++);
    }
  }
});

helpers.slice.js

import Ember from 'ember';

export function slice([input, start, end]) {
  return input ? input.slice(start, end) : null;
}


export default Ember.Helper.helper(slice);

templates.application.hbs

<h1>Welcome to {{appName}}</h1>


{{#each (slice myArray 0 4) as |obj| }}
  {{obj}}
{{/each}}

<button {{action 'remove'}}>Remove</button>
<button {{action 'add'}}>Add</button>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}