protected function _setStateMachine($stateMachine)
    {
        if ($stateMachine !== null && !($stateMachine instanceof StateMachineInterface)) {
            throw $this->_createInvalidArgumentException(
                $this->__('Argument is not a valid state machine instance'),
                null,
                null,
                $stateMachine
            );
        }

        $this->stateMachine = $stateMachine;
    }