Ajax tab container returning Java Script Error

I trying to use a Tab Container control in a asp.net web-page and it's returning 'Object doesn't support this property or method" java script error when opening in browser.  Using .NET 3.5, AjaxControlToolKit 4.1.7.1213 and I.E. 8.  What is wrong with the below code?  Thanks.

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage/Site.Master" AutoEventWireup="true" CodeBehind="ShortSaleInvestorReview.aspx.cs" Inherits="ShortSaleInvestorReview.WebUI.Pages.ShortSaleInvestorReview" %>
<%@ Register Namespace="AjaxControlToolkit" Assembly="AjaxControlToolkit" tagPrefix="ajax" %>

<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<ajax:ToolkitScriptManager ID="scriptmanager1" runat="server">
</ajax:ToolkitScriptManager>

    <ajax:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="1" >
        <ajax:TabPanel runat="server" HeaderText="TabPanel1" ID="TabPanel1">
        <ContentTemplate>
                    <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox><br />
                    <asp:Button ID="Button2" runat="server" Text="Button" />
                </ContentTemplate>
        </ajax:TabPanel>
        <ajax:TabPanel ID="TabPanel2" runat="server" HeaderText="TabPanel2">
        <ContentTemplate>
                    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><br />
                    <asp:Button ID="Button1" runat="server" Text="Button" />
                </ContentTemplate>
        </ajax:TabPanel>
        <ajax:TabPanel ID="TabPanel3" runat="server" HeaderText="TabPanel3">
        <ContentTemplate>
                    <asp:TextBox ID="TextBox3" runat="server"></asp:TextBox><br />
                    <asp:Button ID="Button3" runat="server" Text="Button" />
                </ContentTemplate>
        </ajax:TabPanel>
    </ajax:TabContainer>
  
</asp:Content>