How to send empty timefield?

I'm developing Extjs form and have to send all field, also empty fields. It works for all types of fields only timefield is not send. Here an example with datefield and timefield. Why do they behavior differently? Which config should I use to send empty timefield? Preferred as variable name with empty string as value. Do you have an idea?

Code:

Ext.create('Ext.form.Panel', {
renderTo: Ext.getBody(),
standardSubmit: true,
items: [{
    fieldLabel: 'Datefield',
    format: 'Y-m-d',
    name: 'Datefield',
    // value: '2015-01-01',
    xtype: 'datefield'
},{
    fieldLabel: 'Timefield',
    format: 'H:i:s',
    name: 'Timefield',
    // value: '16:00:00',
    xtype: 'timefield'
}],
buttons: [{
    text: 'Submit',
    formBind: true,
    handler: function(){
        this.up('form').getForm().submit({
            url: 'http://yourfavoriteurl',
            standardSubmit: true,
            method: 'POST' // 'GET'
        });
    }
}] 
});

How to send empty timefield?