title: Undefined is not a function with concat
body: <p>I'm rendering two models on a page with backbone and handlebars. I'm trying to concatenate the two models together but the <code>concat()</code> function returns <code>error:Undefined is not a function</code>.</p>
  
  <p>This is my render function:</p>
  
  <pre><code>render: function (eventName) {
       var twitter=this.collection.models[0].toJSON();
       var instagram=this.collection.models[1].toJSON();
       var conc=twitter.concat(instagram);  &lt;--LINE ERROR      
       $(this.el).html(this.template(conc));
       return this;
  },
  </code></pre>