DataTable with server side with C#

I am trying to load DataTable from server side code is in C# and is doing from webservice. I am getting proper jSON. But I am getting error length is undefined.
My jSON output

{"sEcho":"1","aoData":[{"civilID":"123456789012","firstName":"Adel","lastName":"Alshammeri","mobile":"99466444","parentID":"","rowNum":1},{"civilID":"267122700454","firstName":"Adel","lastName":"S","mobile":"99466444","parentID":"","rowNum":2},{"civilID":"275030107784","firstName":"Mahfuz","lastName":"S","mobile":"66390129","parentID":"267122700454","rowNum":3},{"civilID":"265103002897","firstName":"Antonio","lastName":"S","mobile":"99073592","parentID":"267122700454","rowNum":4},{"civilID":"272110203967","firstName":"Reco","lastName":"S","mobile":"50956026","parentID":"267122700454","rowNum":5},{"civilID":"259061403367","firstName":"Eliseo","lastName":"S","mobile":"50956026","parentID":"275030107784","rowNum":6},{"civilID":"265020307919","firstName":"Liza","lastName":"S","mobile":"66642749","parentID":"275030107784","rowNum":7},{"civilID":"244101000772","firstName":"AlAsmar","lastName":"S","mobile":"99089648","parentID":"275030107784","rowNum":8},{"civilID":"273100700825","firstName":"Alhadad","lastName":"S","mobile":"60993977","parentID":"265103002897","rowNum":9},{"civilID":"268082304389","firstName":"AILEEN","lastName":"s","mobile":"66481975","parentID":"265103002897","rowNum":10}],"iTotalRecords":"42","iTotalDisplayRecords":"42"}

and My javascript code is

$(document).ready(function () {
  $("#org").jOrgChart({
    chartElement: '#chart',
    dragAndDrop: false
  });
  $('#members').dataTable({
    "bServerSide": true,
    "sAjaxSource": "CivilID.aspx/getData",
    "sAjaxDataProp": "",
    "bProcessing": true,
    "bDestroy": true,
    "fnServerData": function (sSource, aoData, fnCallback) {
      $.ajax({
        "dataType": 'json',
        "contentType": "application/json; charset=utf-8",
        "type": "GET",
        "url": sSource,
        "data": aoData,
        "success": function (msg) {
          var json = jQuery.parseJSON(msg.d);
          fnCallback(json);
          //$("#members").show();
        }
      });
    },
    "aoColumns": [
        { "sName": "civilID" },
        { "sName": "firstName" },
        { "sName": "lastName" },
        { "sName": "mobile" },
        { "sName": "parentID" },
        { "sName": "rowNum" }
    ]
  });
});

Kindly can somebody help me what I am doing wrong