Extjs4.1 - Submit Valid form within hidden field has allowblank false

Extjs4.1 - Submit Valid form within hidden field has allowblank false

I have a form panel with dynamic items. Some items has hidden like example:
items: [{
xtype: 'textfield',
fieldLabel: 'Field 1',
name: 'theField'
},{
xtype: 'textfield',
fieldLabel: 'Field 2',
name: 'theField'
},{
xtype: 'textfield',
fieldLabel: 'Field 3',
name: 'theField',
hidden: true,
allowBlank : false
}]
But when i submit my form like
if (form.isValid()) {
alert('submit');
}else alert('fail');
that will check all field, and my form will not submit.
Has anyway to valid form (only field is shown) ? how to do that thanks
Here is my example to check http://jsfiddle.net/jZYcQ/