Gracias a Oleg, mi jqGrid ahora se ve así y funciona bien. (mi problema después del código)

var columnModel = [{ name: 'ID', index: 'ID', sortable: true, summaryType:'count', summaryTpl:'<b>{0} Item(s)</b>' },
{ name: 'FirstName', index: 'FirstName', sortable: true},
{ name: 'LastName', index: 'LastName', sortable: true }
];
var columnNames = ['Id', 'First Name', 'Last Name'];

myGrid.jqGrid({
    url: './WebService.asmx/ViewNQueryData',
    datatype: 'json',
    mtype: 'POST',
    ajaxGridOptions: { contentType: 'application/json; charset=utf-8' },
    serializeGridData: function (postData) {
        if (postData.filters === undefined) postData.filters = null;
        return JSON.stringify(postData);
    },
    jsonReader: {
        root: function (obj) { return obj.d.rows; },
        page: function (obj) { return obj.d.page; },
        total: function (obj) { return obj.d.total; },
        records: function (obj) { return obj.d.records; }
    },
    colModel: columnModel,
    colNames: columnNames,
    rowNum: 10,
    rowList: [10, 20, 300],
    sortname: 'ID',
    sortorder: "asc",
    sortable: true,
    pager: "#ViewNQueryPager",
    viewrecords: true,
    gridview: true,
    height: 250,
    shrinkToFit: true,
    grouping: true,
    groupingView: {
        groupField: ['ID'],
        groupColumnShow: [false],
        groupText: ['<b>{0} - {1} Item(s)</b>'],
        groupSummary: [true],
        groupOrder: ['asc'],
        groupDataSorted: true,
        showSummaryOnHide: true
    },
    footerrow: true,
    userDataOnFooter: true,
    gridComplete: function () {
        $('#totalRecordsFound').html(myGrid.jqGrid('getGridParam', 'records') + " Customers");
    },
    onSelectRow: function () { alert("selected"); }
}).jqGrid('navGrid', '#ViewNQueryPager', 
                { edit: false, add: false, del: false, search: true, view: true },//option
                {}, // use default settings for edit
                {}, // use default settings for add
                {}, // delete instead that del:false we need this
                {multipleSearch: true, multipleGroup: true, showQuery: true, onSearch: function (response) { showQueryDetails(); } },
                {height: 250, jqModal: false, closeOnEscape: true} // view options
                );
myGrid.jqGrid('filterToolbar', { stringResult: true, searchOnEnter: true });

Por alguna extraña razón, puedo seleccionar cualquier fila (todo lo demás funciona bien) y obtengo en el navegador esta excepción:

Esto ocurre solo en Chrome, Firefox funciona bien.

Uncaught TypeError: Cannot call method 'indexOf' of undefined
$.jgrid.extend.setSelection
e.extend.eachjquery-1.6.2.min.js:16
e.fn.e.eachjquery-1.6.2.min.js:16
$.jgrid.extend.setSelection
$.fn.jqGrid
$.fn.jqGrid.each.$.before.click.bind.ts.p.datatype
f.event.handlejquery-1.6.2.min.js:17
f.event.add.i.handle.k

Necesito algunos ojos nuevos para ayudarme y ver qué hice mal ... esto realmente me está volviendo loco ...

Gracias por adelantado

EDITA

Mis datos JSON son como:

{"d":{"__type":"JqGridData","total":3,"page":1,"records":24,"rows":[{"id":1,"cell":["1","Prabir","Shrestha"]},{"id":2,"cell":["2","Scott","Gu"]},{"id":3,"cell":["3","Scott","Gu"]},{"id":4,"cell":["4","Bill","Gates"]},{"id":5,"cell":["5","Steve","Ballmer"]},{"id":1,"cell":["1","Prabir","Shrestha"]},{"id":2,"cell":["2","Scott","Gu"]},{"id":3,"cell":["3","Scott","Hanselman"]},{"id":4,"cell":["4","Bill","Hanselman"]},{"id":5,"cell":["5","Steve","Ballmer"]}]}}