XMLDocument GetElementByID returning null

Hi guys. I’m trying to get an element of a xml by using the function GetElementById, but the function is always returning null.

//get xml text from a web service
string xml = aS.createTree();
XmlDocument tree = new XmlDocument();

//get all nodes with the tag name "item"
XmlNodeList node = tree.GetElementsByTagName("item");
//just for test to see if i could get the attribute value which returns the expected
string idTemp = node[0].Attributes["ID"].Value;
XmlElement elem = tree.GetElementById("1");

elem is always returning null.
Can you guys help me out?

By the way this is the xml that i’m trying to parse:

<?xml version="1.0" encoding="utf-8"?>
   <item id="1">
       <item id="2"></item>
   <item id="5">
       <item id="6"></item>
       <item id="7">
           <item id="8">
               <item id="10">
                   <item id="11"></item>
           <item id="9"></item>

Thanks Graham, already solved it there.
This was the answer, if anybody has the same problem:

I added a doctype to refer my attribute “id” as an ID and the value of the id cannot start with a number, like the following example.

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE tree [
   <!ELEMENT tree ANY>
   <!ELEMENT item ANY>
   <!ATTLIST item id ID #REQUIRED>
   <item id="id_1">
       <item id="id_2"></item>
   <item id="id_5">
       <item id="id_6"></item>
       <item id="id_7">
           <item id="id_8">
               <item id="id_10">
                   <item id="id_11"></item>
           <item id="id_9"></item>