Extends XML Build-in class

The following is my code …
I want the extends the XML class to my custom class(XMLReader),
But i find that i cannot get the firstChild after success load the XML .
Please help ~!
What’s wrong for my coding ~!

[font=Courier New][color=green]function XMLReader() {
}
// Construct an XML object to hold the server’s reply
XMLReader.prototype = new XML();
//ignore XML white space
XMLReader.prototype.ignoreWhite = true;
// this function triggers when an XML packet is received from the server.
XMLReader.prototype.load = function(success) {
if (success) {
trace(“SUCCESS”);
[color=darkred]trace(this);
[/color] } else {
trace(“FAILED”);
}
};
XMLReader.prototype.getRoot = function() {
[color=darkred]return super.firstChild;
[/color]};[/color][/font]
[font=Courier New][/font]



[font=Courier New][color=green]
XMLReader = new XMLReader();
XMLReader.load("[/color][/font][font=Courier New][color=green]http://localhost:8103/Category.xml[/color][/font][font=Courier New][color=green]");
trace(XMLReader.getRoot());

[/color][/font]