Created Unassigned: AutoExtender in July/Sept 2013 Toolkit not working [27581]

The AutoCompleteExtender in the July and September 2013 toolkits always throw this error when the page first opens:

Microsoft JScript runtime error: 'Sys.Extended.UI' is null or not an object

I don't think it's my code because that works fine with older toolkits.

Thanks in advance for any insight or fix.