public function assertArrayOfStrings($config, $key)
    {
        if (!array_key_exists($key, $config)) {
            return $this;
        }
        if (!is_array($config[$key])) {
            throw new ConfigurationException($key." section must be an array.");
        }
        foreach ($config[$key] as $value) {
            if (!is_string($value)) {
                throw new ConfigurationException("Values in the `{$key}` must be strings"
                    . gettype($value) . " found.");
            } elseif ($value==="") {
                throw new ConfigurationException("Empty string not allowed in `{$key}` array.");
            }
        }
        return $this;
    }