I inherit from stock.picking.out to add some fields

_inherit = 'stock.picking.out'

_columns = {
    'transp_name':fields.char('Chauffeur', size=25,store=True),
    'transp_tel':fields.char('Téléphone',size=14,store=True,),
    'transp_cin': fields.char('CIN Chauffeur', size=8,store=True, ),
    'transp_matricule': fields.char('Matricule Camion', size=64,store=True,  ),
}
_defaults = {
    'transp_name':'/',
    'transp_cin': '/',
    'transp_matricule': '/',
}

_order = "origin desc"

and the view :

     <record id="view_inherits" model="ir.ui.view">
        <field name="name">inherits.form</field>
        <field name="type">form</field>
        <field name="model">stock.picking.out</field>
        <field name="inherit_id" ref="stock.view_picking_out_form"/>
        <field name="arch" type="xml">
            <field name="invoice_state" position="after">
              <field name="transp_name"/>
              <field name="transp_cin"/>
              <field name="transp_tel"/>
              <field name="transp_matricule"/>
            </field>
        </field>
    </record>

when I clic in the Save button the transp_* became '' NOT SAVING !