title: OpenLayers3 overlay causes TypeErrors
body: <p>I'm trying to add an overlay layer to my map from some dynamically constructed data.</p>
  
  <pre><code>map.addLayer(new ol.Overlay({
      position: locations[i],
      positioning: 'center-center',
      element: document.getElementById(screen.id)
  }));
  </code></pre>
  
  <p>I'm assuming there is something wrong with the id of the element I'm passing as screen.id but that is an assumption because when I try it this starts spamming the console every time I attempt to interact with my now blank map.</p>
  
  <pre><code>ol.js:186 Uncaught TypeError: a.bf is not a function
  (anonymous function) @ ol.js:186
  l.forEach @ ol.js:80
  l.bf @ ol.js:186
  l.xo @ ol.js:407
  (anonymous function) @ ol.js:394
  </code></pre>
  
  <p>None of which means much to me.  Can anyone shed light on what the error is trying to tell me?</p>