jQuery Grid Plugin - jqGrid

I'm trying to set-up a grid which only does client-side editing (no Ajax back to the server). The data is loaded via Ajax and I've set cellsubmit: 'clientArray' - however, if I leave editurl empty it gives me an error about the Url being required. If I fill it in with an empty page, it "appears" to work. But if you do something like:

- Add new record

- Select record you just added and click on Edit

- Edit the record / click Save.

The client-side record isn't changed.

Here's what the code looks like:

<script>

    $(document).ready(function () {

        $("#grdInvoice").jqGrid({
            url: 'InvoiceHandler.ashx?Method=GetInvoice',
            datatype: "json",
            colNames: ['Category', 'Part #', 'Description', 'Cost', 'PK'],
            colModel: [
                { name: 'Category', index: 'Category', width: 110, editable: true, edittype:"select", editoptions: "A:One;B:Two" },
                { name: 'PartNumber', index: 'PartNumber', width: 170, editable: true, editoptions: { size: 20} },
                { name: 'Description', index: 'Description', width: 200, editable: true, editoptions: { size: 30} },
                { name: 'Cost', formatter: 'currency', align: "right", formatoptions: { prefix: "$" }, index: 'Cost', width: 95, editable: true, editoptions: { size: 10} },
                { name: 'PK', hidden: true, editable: false}
            ],
            caption: "Invoice",
            viewrecords: true,            
            cellsubmit: 'clientArray',
            editurl: "InvoiceHandler.ashx?Method=Nothing"
        });

        $("#btnAdd").click(function () {
            $("#grdInvoice").jqGrid('editGridRow', "new", { reloadAfterSubmit: false });
        });

        $("#btnEdit").click(function() {
            var selRow = $("#grdInvoice").jqGrid("getGridParam", "selrow");
            if (selRow != null)
                $("#grdInvoice").jqGrid("editGridRow", selRow);
            else
                alert("Please select a row.");

        });

        $("#btnDelete").click(function() {
            var selRow = $("#grdInvoice").jqGrid("getGridParam", "selrow");
            if (selRow != null)
                $("#grdInvoice").jqGrid("delGridRow", selRow);
             else
                alert("Please select a row.");
        });
    });
    </script>