title: Attempting to use index type '-1' where index types are not allowed (1 or -1 only)
body: <p>I am using:</p>
  
  <pre><code>$db-&gt;collection-&gt;find()-&gt;sort(array('username' =&gt; '-1'));
  </code></pre>
  
  <p>And I keep getting the error:</p>
  
  <blockquote>
    <p>MongoCursorException: localhost:27017: Attempting to use index type
    '-1' where index types are not allowed (1 or -1 only).</p>
  </blockquote>
  
  <p>I am using MongoDB 2.4.1 with PHP driver 1.3.x.</p>
  
  <p>Why is this?</p>