ExtJS TreePanel doesn't work in IE 8

I have defined a class which represents a tree structure

GeographicalUnits = Ext.extend(Ext.tree.TreePanel, {
    title: 'Регион / Город',
    //rootVisible: false,
    initComponent: function () {
        this.root = {
            text: 'Tree Node',
            expanded: true
        };
        this.loader = {
            url: 'API/GeographicalUnits/GetNodes'
        };
        GeographicalUnits.superclass.initComponent.call(this);
    }
});

I create an instance this way:

Ext.getCmp('MainTabs').openTab(new GeographicalUnits({
                                     id: 'GeographicalUnitsCatalog'
                                 }));

server response is:

开发者_C百科[{"text":"geo1","children":[{"text":"sub1","leaf":true},{"text":"sub2","leaf":true}]}]

it works in Chrome, firefox and Opera, but doesn't work in IE 8. why?

It was some bug in IE. Strange, but installation of HttpWatch fixed the problem.