problems with xpath evaluate with namespaces

I'm trying to evaluate a simple xpath expression.

I have an xml object,

var xn = response.resultXML; 

xn.outerHTML evaluates to:

<Result xmlns="http://url.com/services">
<AE>0</AE>
<FM>0</FM>
<OCX>0</OCX>
<TX>0</TX>
<AR>0</AR>
<Items>
    <RI>
        <PR>1</PR>
        <Date>2003-07-19T00:00:00</Date>
        <AR>228217.5600</AR>
        <AQ>19018.1300</AQ>
        <CBZ>13.519997630331753554502369668</CBZ>
        <XN>1.1266664691943127962085308057</XN>
        <AM>19018.1300</AM>
        <Unit>PerMonth</Unit>
        <UnitString>$/Month</UnitString>
        <DD>0.0</DD>
        <CR>0</CR>
    </RI>
</Items>
</Result>

I'm trying to evaluate this xml using the following query. The xml is the result of a soap webservice call.

var xdoc = xn.ownerDocument;
resolver = xdoc.createNSResolver(xdoc.documentElement);
var es = xdoc.evaluate("Items", xn, resolver, XPathResult.FIRST_ORDERED_NODE_TYPE, null);
var node = es.singleNodeValue

I do not get any results. Can somebody point me in the right direction to retrieve the Items node? Thanks.

EDIT -

This is the outer HTML for xn.ownerDocument.documentElement :

<soap:Envelope xmlns:soap="http://ift.tt/sVJIaE" xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY">
<soap:Body>
    <Result xmlns="http://url.com/services">
        <AE>0</AE>
        <FM>0</FM>
        <OCX>0</OCX>
        <TX>0</TX>
        <AR>0</AR>
        <Items>
            <RI>
                <PR>1</PR>
                <Date>2003-07-19T00:00:00</Date>
                <AR>228217.5600</AR>
                <AQ>19018.1300</AQ>
                <CBZ>13.519997630331753554502369668</CBZ>
                <XN>1.1266664691943127962085308057</XN>
                <AM>19018.1300</AM>
                <Unit>PerMonth</Unit>
                <UnitString>$/Month</UnitString>
                <DD>0.0</DD>
                <CR>0</CR>
            </RI>
        </Items>
    </Result>
</soap:Body>
</soap:Envelope>