Search elements with XPath

Hi,

I have this people.xml:

<?xml version="1.0" encoding="UTF-8"?>
<wsdl:definitions 
targetNamespace="http://localhost:8080/wsapp/services/PeopleService" 
xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/">
   <wsdl:portType name="PeopleService">
     <wsdl:operation name="add" parameterOrder="name sex age"/>
     <wsdl:operation name="getList"/>
     <wsdl:operation name="del" parameterOrder="id"/>
   </wsdl:portType>
</wsdl:definitions>

How can I find all wsdl:operation which are childs of wsdl:portType 
name="PeopleService"?

I wrote:

var doc = document.implementation.createDocument("","",null);
doc.async = false;
doc.load("people.xml");
var services = 
doc.evaluate("/definitions/portType@name='PeopleService'/operations", 
doc, "http://schemas.xmlsoap.org/wsdl/", XPathResult.ANY_TYPE, null);
while (services.iterateNext()) {
     alert(operation[i].getAttribute("name"));
     alert(operation[i].getAttribute("parameterOrder"));
}

But I got:

Error: uncaught exception: [Exception... "Could not convert JavaScript 
argument arg 2 [nsIDOMXPathEvaluator.evaluate]" nsresult: "0x80570009 
(NS_ERROR_XPC_BAD_CONVERT_JS)" location: "JS frame :: 
file:///home/john/devel/xul/wsdl/wsdl.js :: analize :: line 4" data: no]

What's wrong?

--
Thanks,
Eugene Prokopiev