I'm trying to add inheritance on existing object in Odoo, which is "portal.mixin" into "repair.order" object.

I tried with this code:

class RepairOrder(models.Model):
    _name = 'repair.order'
    _inherit = ['portal.mixin']

but I got this error :

File "/usr/lib/python3/dist-packages/odoo/fields.py", line 3114, in _setup_regular_full
    invf = comodel._fields[self.inverse_name]
    KeyError: 'invoice_id' - - -

Any help please ?

Thanks.