title: Highstock + Highmaps not rendering
body: <p>I haven't been able to get my map to show any data; it just renders blank and in the console there's an error of "Uncaught TypeError: Cannot read property 'length' of undefined". I believe that the versions of Highstock and Highmaps that I use are compatible so I don't think that's the issue, however I could be wrong.</p>
  
  <p><a href="http://jsfiddle.net/4a79s3qp/1/" rel="nofollow">Click here for the fiddle</a></p>
  
  <p><b>JS (geo_data is being fetched with AJAX)</b></p>
  
  <pre><code>$('#map').highcharts('Map', {
      title: {
          text: 'Installs Map'
      },
      colorAxis: {
          min: 0
      },
      series: [{
          data: geo_data,
          mapData: Highcharts.maps['custom/world'],
          joinBy: ['hc-key', 'country_id'],
          name: 'Installs',
          states: {
              hover: {
                  color: '#BADA55'
              }
          }
      }],
      credits: {
          enabled: false
      }
  });
  </code></pre>