0x800a138f - JavaScript runtime error: Unable to get property 'UI' of undefined or null reference

I am trying to apply a calendar extender to a textbox that I have on my form.  My code looks like this:

<asp:TextBox ID="txtstart" runat="server" ></asp:TextBox>
            <ajaxToolkit:CalendarExtender ID="extendertest" runat="server" TargetControlID="txtstart"></ajaxToolkit:CalendarExtender>

It compiles but when I hit run, this is what happens:

0x800a138f - JavaScript runtime error: Unable to get property 'UI' of undefined or null reference

Thoughts?