Greetings!

I'm getting a GridFilters isn't a Constructor Exception in FireFox by starting this code... Any idea what's wrong?

Code:
Ext.namespace('Item', 'Item.list', 'Item.functions');

Ext.onReady(function()         
{
	Item.list.domainID=-10;
	Item.list.operatorID=-10;
	Item.list.subjectID=-10;
		
	Item.list.items = new Ext.data.JsonStore
	({
		url:'../../ItemListController',
		root: 'results',		
		totalProperty: 'total',		
		fields: 
		[
			{name:'id'},	      
			{name:'text'},     
			{name:'type'},
			{name:'statistic'},
			{name:'lastUsed', type: 'date', dateFormat: 'Y-m-d'}			
		],
		
		sortInfo: {field: 'id', direction: 'ASC'},
		remoteSort: true
	});

      var filters = new Ext.grid.GridFilters({
        filters:[
        {type: 'numeric',  dataIndex: 'id'}
       ]}); 
	
	Item.list.items.on('beforeload', function() 
    {
	  Item.list.items.baseParams =
	  { 
	    domainID :Item.list.domainID,
	    operatorID :Item.list.operatorID,
	    subjectID : Item.list.subjectID
	  };
    });    
	
	Item.functions.showItemList = function()
	{						
		Exam.list.examList.hide();
		Item.list.itemList.setVisible(true);
		Item.main.content.doLayout();
	};
	

	
	Item.list.columnModel = new Ext.grid.ColumnModel
	([
		  {dataIndex: 'id', header: 'Item-ID', width:80},
		  {dataIndex: 'text', header: 'Kurztext', width:620},
		  {dataIndex: 'typ', header: 'Typ', width:150},
		  {dataIndex: 'statistic', sortable: false, header: 'Kennzahlen', width:100},
		  {dataIndex: 'lastUsed', sortable: false, header: 'Klausur', width:100}
	]);
	
	Item.list.columnModel.defaultSortable = true;
	
	Item.list.itemList = new Ext.grid.GridPanel
	({
		  id: 'itemList',
		  title: 'test',
		  ds: Item.list.items,
		  cm: Item.list.columnModel,	
		  enableColLock: false,
		  loadMask: true, 
		  bbar: new Ext.PagingToolbar
		  ({
		    store: Item.list.items,
		    pageSize: 34
		  })
	});		
	
	Item.list.itemList = new Ext.grid.GridPanel
	({
	  id: 'itemList',
	  title: 'test',
	  ds: Item.list.items,
	  cm: Item.list.columnModel,	
	  draggable: true,	  
	  enableColLock: false,
	  loadMask: true, 
	  bbar: new Ext.PagingToolbar
	  ({
	    store: Item.list.items,
	    pageSize: 34
	  })
	});	
	Item.list.itemList.setHeight(790);
	
	 
	
});