PHP unable to access Xpath node of a SimpleXMLElement Object

$x = new DomDocument();
$x->loadXML($responseXml);
$xml = simplexml_import_dom($x);

Outputting the array using print_r($xml) gives the following:

SimpleXMLElement Object
(
    [Timestamp] => 2014-11-09T18:28:47.843Z
    [Ack] => Success
    [Version] => 897
    [Build] => E897_UNI_API5_17253832_R1
    [Store] => SimpleXMLElement Object
        (
            [Name] => test
            [SubscriptionLevel] => Basic
            [Description] => Welcome Message.           
        )

)

Using $xml->Store->Description outputs "Welcome Message."

When I use xpath to return the Description node using the following code, I get an empty array:

$xpath = new DOMXPath($x);
$result = $xpath->query("/Store/Description");

Why does this fail?