I am trying to inherit "account.bank.statement.line" model. For this i am writing a below code.

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class account_bank_statement_line(osv.osv):

    _inherit = 'account.bank.statement.line'

    _columns= {

        'rental_statement_id': fields.many2one('rental.vehicle.contract','Contract payment', ondelete='cascade'),

    }

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and when i update my model i get this error : 

return self.models[model_name]

KeyError: 'account.bank.statement.line'