private function validateDirs($config): void
    {
        // Check that dirs key exists
        if (! Validator::key('dirs')->validate($config)) {
            throw new InvalidConfigException('"dirs" parameter is not an array.');
        }
        // Clean dirs value
        if ($config['dirs'] === null) {
            $this->config['dirs'] = $config['dirs'] = [];
        }
        // Validate each dirs
        if (! Validator::arrayVal()
            ->each(Validator::stringType(), Validator::stringType())->validate($config['dirs'])
        ) {
            throw new InvalidConfigException('Some directories in "dirs" parameter are not strings.');
        }
    }