title: Leaflet.js & Mapbox.js - Uncaught TypeError: Object [object Object] has no method 'on'
body: <p>I'm using Mapbox, which is based on Leaflet.js to generate my maps. I'm trying to add a layer group to my map. But every time I call .addTo(map), I get </p>
  
  <pre><code>Uncaught TypeError: Object [object Object] has no method 'on' 
  </code></pre>
  
  <p>My code is as follows:</p>
  
  <pre><code>map = L.mapbox.map("map", "dannyvaughton.map-lmwtrr8k").setView([54.4, 0.43], 14)
  markers = new L.LayerGroup().addTo(map)
  </code></pre>
  
  <p>Thanks.</p>