I created a new object for alternate model in product, but everytime I access products - it returns an error: (I'm using new api)

Error

ValueError: too many values to unpack

Python file

from openerp import fields, models

class ProductAlternateModel(models.Model):
    _name = 'product.alternate.model'

    #fields

class product_product(models.Model):
    _inherit = 'product.product'

    alternate_model_ids = fields.Many2many('product.alternate.model')

XML File (Product Form)

<xpath expr="//field[@name='lst_price']" position="after">
                <notebook>
                    <page string="Alternate Model">
                        <field name="alternate_model_ids"/>
                    </page>
                </notebook>
            </xpath>