AJAX data source throwing TypeError: b[a] is undefined

(debug code ucolef )

I'm trying to get my tables working with an AJAX source (client-side processing). The table definition looks like this:

var bbTable = $('#bb_datatable').DataTable({

            "sDom": "<'row  bottom-buffer10' <'.col-sm-4'><'.col-sm-4'> <'.col-sm-4 pull-right' f>>t<'row'<'.col-sm-8'i r><'pull-right'>S>",
            "columns" : [
                {
                    'data': 'field_id_59',
                    'bVisible' : true
                },
                {
                    'data': 'field_id_60',
                    'bVisible' : true
                }
            ],
            "scrollY": "600px",
            "scrollCollapse": true,
            "paginate": false,
            "order" : [],
            "select": "single",
            "buttons": [],
            "ajax": {
                "url": bb.ajaxPath,
                "data": function ( d ) {
                    return $.extend( {}, d, {
                        "action": "fill-datatable",
                        "query_id": 28,
                        "col_prop_name_array": ['field_id_59', 'field_id_60']
                    } );
                }
            }
        }
    );

The AJAX call returns something that looks identical to the analogous examples on this site:

{
    "data": [{
        "field_id_59": "1",
        "field_id_60": "BigBench Adminx"
    }, {
        "field_id_59": "2",
        "field_id_60": "BigBench Mouse "
    }]
}

...and datatables then throws the above error.

If, instead of "ajax: ", I specify the following, no error is generated:

"data": [{"field_id_59":"1","field_id_60":"Big Bench Adminx"},{"field_id_59":"2","field_id_60":"Big Bench Mouse"}],

Comparison of this problem to some previous posts in this forum suggests that Datatables doesn't like the JSON that is being returned. I know that I used to specify iTotalRecords and iTotalDisplayRecords in the JSON object that was returned but this seems to have been deprecated in 1.9x for client side processing?

Thanks in advance,

Rob