Original function in "account.voucher" was:

    def button_proforma_voucher(self, cr, uid, ids, context=None):
        self.signal_workflow(cr, uid, ids, 'proforma_voucher')
        return {'type': 'ir.actions.act_window_close'}

I have overriden the base class function:

class MyAccountVoucher(models.Model):
    _inherit = "account.voucher"

# Override "Pay" button press method

    @api.one
    def button_proforma_voucher(self):
------ some code that works fine -----
 res = super(MyAccountVoucher,self).button_proforma_voucher
return res

Nothing happens. I assume that the problem may be in the middle between new and old api, but what is wrong in the code?