private function assertExists($type)
    {
        if (!isset($this->typePaths[$type])) {
            throw new Exception\DocumentTypeNotFoundException(
                sprintf(
                    'Structure path for document type "%s" is not mapped. Mapped structure types: "%s"',
                    $type,
                    implode('", "', array_keys($this->typePaths))
                )
            );
        }
    }