70-515 - Ajax Suggested Practices

Hi,

I am currently in the middle of studying for my 70-515 exam (Web Application Development with Microsoft.NET Framework 4). I'm at the chapter studying AJAX, and one of the suggested practices is as follows;

"Create an AJAX client control that extends an ASP.NET button with a confirmation alert window."

Now i've attempted this practice but it doesn't seem to be working. The error I am receiving when trying to run the application is "JavaScript runtime error: 'Sys' is undefined"

I have been using the book for reference when creating the bit of code below, but it still doesn't seem to be working. I was hoping you guys may be able to shed some light on where I am going wrong.

Ajax.js

///<reference name="MicrosoftAjax.js" />

Type.registerNamespace("Ajax");

Ajax.AlertBox = function (element) {
    Ajax.AlertBox.initializeBase(this, [element]);

    Ajax.AlertBox.prototype = {
        initialize: function() {
        Ajax.AlertBox.callBaseMethod(this, 'initialize');

        this._OnClickHandler = Function.createDelegate(this, this._OnClick);
        $addHandlers(this.get_element(), {'click' : this._OnClick}, this);
        },

        dispose: function() {
        $clearHandlers(this.get_element());
        Ajax.AlertBox.callBaseMethod(this, 'dispose');
        },
        _OnClick : function() {
            alert("This box has appeared because the custom button has been clicked!");

        }
        }
        }
        Ajax.AlertBox.registerClass('Ajax.AlertBox', Sys.UI.Control);

        if (typeof(Sys) !== 'undefined') Sys.Application.notifyScriptLoaded();

Default.aspx

<%@ Page Title="Home Page" Language="VB" MasterPageFile="~/Site.Master" AutoEventWireup="false"
    CodeFile="Default.aspx.vb" Inherits="_Default" %><asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent"><script language="javascript" type="text/javascript">
        var app = Sys.Application;
        app.add_init(appInit);

        function appInit(sender, args) {
            $create(Ajax.AlertBox, null, null, null, $get('MainContent_Button1'))}</script></asp:Content><asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent"><asp:ScriptManager runat="server" ID="ScriptManager1"><Scripts><asp:ScriptReference Path="~/Scripts/AjaxEnabled.js" /></Scripts></asp:ScriptManager><asp:Button id="Button1" runat="server" /></asp:Content>

The line of code that this seems to be erroring out on is within the Default.aspx on the line which has var app = Sys.Application.

Any help on this is greatly appreciated as I'm pulling my hair out on this one now!

Thanks,