Custom sorting with angular datatables directive

I am using the angular datatables within my app. So far everything works well, except when I try to add custom sorting.

I have a set of data that returns a hyphen, "-" if there is no data. Here is my sorting function::

     $.fn.dataTableExt.oSort['nullable-asc'] = function(a,b) {

        if (a == '-')
            return 1;
        else if (b == '-')
            return -1;
        else
        {
            var ia = parseInt(a);
            var ib = parseInt(b);
            return (ia<ib) ? -1 : ((ia > ib) ? 1 : 0);
        }
    }

    $.fn.dataTableExt.oSort['nullable-desc'] = function(a,b) {
    console.log(a,b)
        if (a == '-')
            return 1;
        else if (b == '-')
            return -1;
        else
        {
            var ia = parseInt(a);
            var ib = parseInt(b);
            return (ia>ib) ? -1 : ((ia < ib) ? 1 : 0);
        }
    }

    ..start controller...

        vm.dtColumnDefs = [
            DTColumnDefBuilder.newColumnDef(0).notSortable().withClass('hidden-print')
            DTColumnDefBuilder.newColumnDef(1).withClass('td-fieldname'), //name
            DTColumnDefBuilder.newColumnDef(2), //fieldType
            DTColumnDefBuilder.newColumnDef(3).withOption('type', 'html-num-fmt'),
            DTColumnDefBuilder.newColumnDef(4).notSortable(), //distribution
            DTColumnDefBuilder.newColumnDef(5).withOption('type', 'html-num-fmt'), //cardinality
            DTColumnDefBuilder.newColumnDef(6).withOption('type', 'nullable'), //Min
            DTColumnDefBuilder.newColumnDef(7).withOption('type', 'nullable') //Max
        ];

    ...other angular code..... 

This works just fine when tried on a standard table, but doesnt play nice with the angular app. When I click the heading of the table to sort - nothing happens. Console.log above yields blank values. Is there a way to use custom sorting with the angular directive?