Checkbox not checked by default

[senacle]

Hello,

I've a checkbox i want to be checked by default.
But it doesn't work.

My code which doesn't check the checkbox (i give very simple example, without all the config of my formpanel) :

    var my_form = new Ext.FormPanel({
        hidden: 'true',
        items: [{
            columnWidth: 0.55,
            defaults: {labelStyle: 'font-weight:bold; color:#990033;'},
            items: [{
                checked: true,
                xtype: 'checkbox'
            }],
            layout: 'form',
            xtype: 'panel'
        }],
        layout: 'column'
    });

If the hidden property of my_form is set to false, it's OK.
If the hidden property of my_form is set to true, and after with some code i show the my_form, the checkbox appears not checked.