title: Layered navigation with custom attribute
body: <p>We have a layered navigation (<a href="http://cl.ly/image/1V2b2W0a2Y29" rel="nofollow">screenshot</a>) that allows our users to filter by category, price, and colors. category and price work just fine, as they are default to Magento. The issue comes when attempting to filter by color, a custom attribute.</p>
  
  <p>It throws an exception. I was able to track down the stack trace, but I'm unable to make sense of it. Wondering if anyone can help point me in the right direction. Trace: <a href="https://gist.github.com/4490917" rel="nofollow">https://gist.github.com/4490917</a></p>
  
  <p>We're using Magento Enterprise v1.12.0.2</p>