How do see subfolders when selecting files from a folder in Google Docs API?

Question

Google provides the ability to search for folders with specific names. While this does search all folder trees for a folder with that name, I would like to search for a folder name that is already under a particular folder. Yet when I use the code that they show as an example on their site:

function retrieveDocsInNamedFolder($gdClient, $html, $folderName,
  $folderOwnersEmailAddress) {
    if ($html) {echo "<h2>Your starred presentations</h2>\n";}

  $folderCategory = '{http://schemas.google.com/docs/2007/folders/' .
    $folderOwnersEmailAddress . '}' . $folderName;
  $queryUrl = 'http://docs.google.com/feeds/documents/private/full/-/' .
    urlencode($folderCategory);
  $feed = $gdClient->getDocumentListFeed($queryUrl);

  printDocumentsFeed($feed, $html);
}

retrieveDocsInNamedFolder($gdClient, $html, 'starred', $email);

If I provide the name of a folder with only one subfolder inside, the feed is returned empty, leading me to believe that folders are not returned in the query.