here's my method:

@api.multi
def action_invoice_open(self):
    res = super(AccountInvoice, self).action_invoice_open()
    for record in self:
        obj = record.env['account.asset.category'].search(
            [('method_number', '=', record.invoice_line_ids.total_months)], limit=1).id
        if record.invoice_line_ids.deferred_revenue == True:
            for rec in record:
                rec.env['account.asset.asset'].create({
                    'name': rec.name or 'kkkkk',
                    'category_id': obj,
                    'date': rec.date,
                    'value': rec.invoice_line_ids.price_subtotal,
                    'partner_id': rec.partner_id.id,
                    'invoice_id': rec.id,

                })
            return res

it gives this error

ValueError: Expected singleton: account.invoice.line(494, 495, 496)

Error is in the bold line. Any help? 

P.S: I'm getting these values from line and it works fine with single line value but gives error when i add multiple lines.