jQuery Grid Plugin - jqGrid

I'm facing a problem with JSON Data.

I started with jqGrid "Demos Array" (local data) and it works fine.

Now, I would like to move to json callback but the grid shows 'Loading ...' and firebug show this error:

n is undefined

grid.base-min.js :... b.fn.setSelection=function(w,z,q){return this.each(function(){var g=this,m,l,n;z=z===false?false:true;if(w===false){l= ...

I provide "grid_data.txt" file in apache server root folder containing the json data:

{"total": "1","page": "1", "records": "1", "rows" :  [ {"id":"1","invdate":"2007-10-01","name":"test","note":"note","amount":"200.00","tax":"10.00","total":"210.00"}
  ]}

and this is script part of grid_url.html:

<script type="text/javascript">
// We use a document ready jquery function.
jQuery.noConflict();
jQuery(document).ready(function(){
  gridimgpath = "themes/green/images";
  jQuery("#list4").jqGrid({
        datatype: 'json',
        url: 'grid_data.txt',
        mtype: 'GET',
      height: 250,
         colNames:['Inv No','Date', 'Client', 'Amount','Tax','Total','Notes'],
         colModel:[
             {name:'id',index:'id', width:60, sorttype:"int"},
             {name:'invdate',index:'invdate', width:90, sorttype:"date"},
             {name:'name',index:'name', width:100},
             {name:'amount',index:'amount', width:80, align:"right",sorttype:"float"},
             {name:'tax',index:'tax', width:80, align:"right",sorttype:"float"},
             {name:'total',index:'total', width:80,align:"right",sorttype:"float"},
             {name:'note',index:'note', width:150, sortable:false}
         ],
         pager: jQuery('#pager2'),
         imgpath: gridimgpath,
         multiselect: true,
         caption: "Manipulating Array Data",
      jsonReader : { root: "rows", page: "page", total: "total" , records: "records" }
  });
});
</script>

Any thoughts are appeciated. Regards.