Ajax Index Column Error

I want add an index numbering from ajax in my table.
i have follow this documentation http://datatables.net/examples/api/counter_column.html and it give an error "requested unknown parameter '1' from the data source for row 0".
then i try search some solution in forum and do some googling. i make some modification from documentation like these.
my php:

$this->load->library('datatables');
$this->datatables->from('lesson');
$this->datatables->select('lesson_name');
echo $this->datatables->generate();

my javascript:

var get_jurusan = function (table, url) {
    var oTable = $('#'+table).dataTable({
        "sPaginationType": "bs_normal",
        "bProcessing": true,
        "bServerSide": true,
        "sAjaxSource": url,
        "sServerMethod": "POST",
        "aoColumnDefs": [
            { "bSortable": false, "aTargets": [ -1 ] }
        ],
        "aoColumns": [
            {
                "sTitle": "Index",
                "fnDrawCallback": function ( oSettings ) {
                    if ( oSettings.bSorted || oSettings.bFiltered )
                    {
                        for ( var i=0, iLen=oSettings.aiDisplay.length ; i<iLen ; i++ )
                        {
                            $('td:eq(0)', oSettings.aoData[ oSettings.aiDisplay[i] ].nTr ).html( i+1 );
                        }
                    }
                }
            },
            { "sTitle": "Lesson", "fnRender": function( oObj ) { return oObj.aData[0]} }
        ],
        "aLengthMenu": [
            [1, 10, 15, 20, -1],
            [1, 10, 15, 20, "Semua"]
        ],
        "iDisplayLength": 10,
    });
    wrapper(table);
};

function wrapper(table){
    $('#'+table+'_wrapper .dataTables_filter input').addClass("form-control input-sm").attr("placeholder", "Cari...");
    $('#'+table+'_wrapper .dataTables_length select').addClass("m-wrap small");
    $('#'+table+'_wrapper .dataTables_length select').select2();
}

Ajax response:

{"sEcho":0,"iTotalRecords":3,"iTotalDisplayRecords":3,"aaData":[["Mathematic"],["Basic Algorithm"],["Java"]],"sColumns":"lesson_name"}

but this make the lesson data show in both column like these http://oi40.tinypic.com/2lasb43.jpg.

this is link for debug result http://debug.datatables.net/ufazow

please help