XPath无效表达式

<?php
  $feed = file_get_contents('http://thexmofo.wordpress.com/feed/');
  $xml = new SimpleXMLElement($feed);
  $xml->registerXPathNamespace('media', 'http://thexmofo.wordpress.com/feed/');
  $images = $xml->xpath('/rss/channel/item/media:content@url');
  var_dump($images);
?>

有人能告诉我为什么我 Warning: SimpleXMLElement::xpath() [simplexmlelement.xpath]: Invalid expression bool(false) ?