setActiveItem and string/int

gistfile1.js

var cnt = new Ext.Container({
    fullscreen : true,
    layout     : 'card',
    items      : [
        {
            xtype  : 'toolbar',
            docked : 'top',
            items  : [
                {
                    text    : 'Test (int)',
                    handler : function (button) {
                        cnt.setActiveItem(2);
                    }
                },
                {
                    text    : 'Test (string) (error in console)',
                    handler : function(button) {
                        cnt.setActiveItem('2');
                    }
                }
            ]
        },
        {
            html   : 'First Page'
        },
        {
            html   : 'Second Page'
        },
        {
            html   : 'Third Page'
        }
    ]
});