protected function validateAliasEntry($entry)
    {
        if (!is_string($entry)) {
            $type = is_object($entry) ? get_class($entry) : gettype($entry);
            $message = sprintf('Argument "entry" in "%s" must be a string; "%s" was given.', __METHOD__, $type);
            throw new Exception\InvalidArgumentException($message);
        }

        if (!$this->has($entry)) {
            throw new Exception\NotFoundException($entry);
        }
    }