XMLDocument GetElementByID returning null
===

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();
    tree.LoadXml(xml);

    //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" ?>
    <!DOCTYPE tree [
       <!ELEMENT tree ANY>
       <!ELEMENT item ANY>
       <!ATTLIST item id ID #REQUIRED>
    ]>
    <tree>
       <item id="1">
           <item id="2"></item>
       </item>
       <item id="5">
           <item id="6"></item>
           <item id="7">
               <item id="8">
                   <item id="10">
                       <item id="11"></item>
                   </item>
               </item>
               <item id="9"></item>
           </item>
       </item>
    </tree>