Can't access object values by dynamic key in Handlebars

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  numbers:['1','2','3','4'],
  letters:['a','b','c','d']
});

application.template.hbs

{{#each numbers as | value index |}}
	{{my-component 
  	number=value
    letter=(get-at-index letters index)
  }}
{{/each}}

helpers.get-at-index.js

import Ember from 'ember';

export function getAtIndex(params/*, hash*/) {
  return params;
}

export default Ember.Helper.extend({
  recomputeOnArrayChange: Ember.observer('_array.[]', function() {
  	this.recompute();  
  }),
  
  compute: function([array, index]) {
		this.set('_array', array);
    
		return array.objectAt(index);
  }
});

my-component.template.hbs

{{letter}} is letter number {{number}}

twiddle.json

{
  "version": "0.4.17",
  "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/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}

See also: http://discuss.emberjs.com/t/iterate-over-a-list-use-index-to-fetch-item-from-another-list-pass-in-to-nested-component/9310/2