hers's my code:

class ItemIssueReturn(models.Model):
    _inherit = 'item.issue.return'
    helpdesk_id = fields.Many2one('helpdesk.ticket', readonly=True)
    maintenance_id = fields.Many2one('maintenance.request', readonly=True)

    @api.multi
    def action_confirm(self):
        res = super(ItemIssueReturn,self).action_confirm()
        if self.maintenance_id.id:
            self.maintenance_id.state = 'inventory_process_done'
        self.maintenance_id.state = 'inventory_process_done'
        return res


here's the error
File "/home/syedhamza/custom/programming/enviornments/flectra/thirdparty/maintenance/maintenance_ext/models/item_issue_return.py", line 13, in action_confirm
    res = super(ItemIssueReturn,self).action_confirm()
  File "/home/syedhamza/custom/programming/enviornments/flectra/thirdparty/financials/cwip/models/models.py", line 188, in action_confirm
    result = super(ItemIssueReturnCwipExt, self).action_confirm()
  File "/home/syedhamza/custom/programming/enviornments/flectra/thirdparty/scm/stock_issue_return/models/models.py", line 163, in action_confirm
    raise ValidationError(_(line.product_id+' No product price at date!'))
  File "/home/syedhamza/custom/programming/enviornments/flectra/src/flectra/flectra/models.py", line 4645, in __add__
    return self.concat(other)
  File "/home/syedhamza/custom/programming/enviornments/flectra/src/flectra/flectra/models.py", line 4654, in concat
    raise TypeError("Mixing apples and oranges: %s.concat(%s)" % (self, arg))
TypeError: Mixing apples and oranges: product.product(3280,).concat( No product price at date!)