title: Leaflet Markercluster - "this._featureGroup.addEventParent is not a function"
body: <p>I am adding <a href="https://github.com/Leaflet/Leaflet.markercluster" rel="noreferrer">Leaflet.markercluster</a> plugin to my existing <a href="http://leafletjs.com/" rel="noreferrer">Leaflet</a> map.</p>
  
  <p>I am getting this error:</p>
  
  <blockquote>
    <p>Uncaught TypeError: <code>this._featureGroup.addEventParent</code> is not a function</p>
  </blockquote>
  
  <p>In relation to the line <code>var locations = L.markerClusterGroup();</code>.</p>
  
  <p>I cannot find any information about what this error might mean.</p>
  
  <p>I am linking to the Markercluster JS file, and after Leaflet. Not sure why else it would not be able to find a function.</p>