protected function getClassParameter()
    {
        $class = parent::getClassParameter();
        $rootUserClass = $this->getClass(GenerisRdf::CLASS_GENERIS_USER);

        if ($class->getUri() == $rootUserClass->getUri()) {
            return $class;
        }

        /** @var core_kernel_classes_Class $instance */
        foreach ($rootUserClass->getSubClasses(true) as $instance) {
            if ($instance->getUri() == $class->getUri()) {
                return $class;
            }
        }

        throw new InvalidArgumentException('Only user classes are allowed as classUri.');
    }