Extjs: XmlReader record mapping problem

The problem is described here.

Ext.onReady(function(){
   var readerTop10 = new Ext.data.XmlReader({
       record: 'top10_product'
       }, [
       {name: 'unid', mapping: '@id'},
       {name: 'product', mapping: new String()}
       ]);

   var storeTop10 = new Ext.data.Store({
       url: 'http://someurl.xml',
       reader: readerTop10  
   });

   var gridTop10 = new Ext.grid.GridPanel({
       store: storeTop10,
       columns: [
           {header: "ID", dataindex:'unid'},
           {header: "Product", dataindex: 'product'}
       ],
       renderTo:document.body,
       loadMask: true,
       title:'Top 10 producten',
   });

   storeTop10.load();
});