Error with Ajax Extensions for asp.net 4.0

Error with Ajax Extensions for asp.net 4.0

I'm trying to get AJAX extensions working on an existing Web Forms
Application.
I've added the AjaxControlToolkit using Nuget. As a test I setup a page
with a button and added a button confirmation extension.
I'm getting this error:
0x800a138f - JavaScript runtime error: Unable to get property 'UI' of
undefined or null reference
On the following line in the dynamically created Javascript:
$create(Sys.Extended.UI.ConfirmButtonBehavior, {"ConfirmText":"Are ya
totally sure?","id":"MainContent_Button1_ConfirmButtonExtender"}, null,
null, $get("MainContent_Button1"));
I'm guessing that one of the libraries is the wrong version, but don't
know how to go about figuring this out. I've tried to update packages
using Nuget.
Any ideas?
Thanks!