Node.prototype.selectNodes function problems

I'm trying to create a javascript application that uses data returned
from a (ASP.NET) webservice. I want a cross-browser (IE/Mozilla)
solution for this. The webservice data loads fine into a document, but
then I want to use XPath to select particular nodes in this document.
IE has a selectNodes function for this, so I want to create this
function for Mozilla as well. I have the following for Mozilla:

Node.prototype.selectNodes = function(sExpr)
{
  //DOCUMENT_NODE = 9
  var doc = this.nodeType == 9 ? this : this.ownerDocument;

  var nsRes = doc.createNSResolver(this.nodeType == 9 ?
this.documentElement : this);

  var xpRes = doc.evaluate(sExpr, this, nsRes, 5, null);

  var res = [];
  var item;
  while (item = xpRes.iterateNext())
    res.push(item);
  return res;
}

When I use the Mozilla javascript debugger on this piece of code and
the doc.evaluate statement has been executed, the xpRes variable
contains a lot of XPathExceptions, with the following value: "The
expression cannot be converted to return the specified type."

Any help appreciated