title: Rails: undefined method `>' for nil:NilClass when calling time_ago_in_words
body: <p>I have some simple code in a partial for one of my views that iterates through a <code>comments</code> collection:</p>
  
  <pre><code>&lt;%= time_ago_in_words(comment.created_at) %&gt;
  </code></pre>
  
  <p>returns a <code>undefined method &gt;' for nil:NilClass</code> error.</p>
  
  <p>Meanwhile, </p>
  
  <pre><code>&lt;%= comment.created_at %&gt;
  </code></pre>
  
  <p>works fine and returns:</p>
  
  <p><code>2013-12-25 12:19:53 UTC</code></p>
  
  <p>Any ideas? Thanks.</p>