jqgrid undefined integer? pager not loading

IM using jqgrids on mvc4, I need to get a simple list and display it using Ajax.

When I load the page then grid starts an Ajax call, I have only 2 columns on grid, user_id and name.

When the Json is loaded I get the next error on Google chrome:

Uncaught Typesetter: Cannot read property 'integer' of undefined

and in firefox, firebug:

TypeError: b.jgrid.formatter is undefined

on jquery.jqGrid.src.js:122

And the grid shows an "undefined" msg, also, the current pageer control isnt loading, but the data is

 <table id="GridUsuarios"></table>
    <div id="PagerUsuarios"></div>


  <script type="text/javascript"> 
        $(document).ready(function() {
            jQuery("#GridUsuarios").jqGrid({
                url: '@Url.Action("UsuariosGridData","Usuarios")',
                datatype: "json",
                myType: 'GET',
                contentType: "application/json; charset-utf-8",                
                colNames: ['Usuario', 'Nombre'],
                colModel: [
                    { name: 'user_id', index: 'user_id', width: 90},
                    { name: 'nombre', index: 'nombre', width: 200}
                ],
                pager: '#PagerUsuarios',
                rowNum: 10,
                rowList: [10, 20, 30],
                viewrecords: true,
                height: 'auto',
                sortname: 'nombre',
                sortorder: 'desc',
                caption: "Usuarios",
                jsonReader: {
                    root: "rows",
                    total: "total",
                    page: "page",
                    records: "records",
                    repeatitems: false,
                    id: "user_id"
                },
            });            
        });
    </script>

and my controller:

    public JsonResult UsuariosGridData(string sidx = "