title: Sorting NSOrderedSet using NSSortDescriptor
body: <p>When executing this code:</p>
  
  <pre><code>NSSortDescriptor *sortDescriptor = [Characteristic sortDescriptor];
  [workingSet sortUsingComparator:[sortDescriptor comparator]];
  </code></pre>
  
  <p>I get this error:</p>
  
  <pre><code>*** -[NSMutableOrderedSet sortUsingComparator:]: comparator cannot be nil
  </code></pre>
  
  <p><code>sortDescriptor</code> is non-nil, so I've no idea why this doesn't work.</p>
  
  <p>I can work around the problem with the code below, which works perfectly:</p>
  
  <pre><code>NSSortDescriptor *sortDescriptor = [Characteristic sortDescriptor];
  NSArray *workingArray = [[workingSet array] sortedArrayUsingDescriptors:[NSArray arrayWithObject:sortDescriptor]];
  workingSet = [NSMutableOrderedSet orderedSetWithArray:workingArray];
  </code></pre>