Custom sorting plugins error "b is null"

flashfog

I'm trying to implement the custom sorting plugin following the documentation at http://datatables.net/plug-ins/sorting.
When I specify just "sType": "numeric-comma" I get an error "b is null" for any sorting plugin I specify.

If I try "sSortDataType": "numeric-comma", I get no error but nothing seems to happen with the sorting.

Trying "sSortDataType": "numeric-comma", "sType": "numeric" gives no error but sorting doesn't work as intended.

So which is the proper method? The documentation and examples show all 3 methods being used with no explanation of the difference.

Thanks.