code: public function assertString($config, $key)
      {
          if (array_key_exists($key, $config) && !is_string($config[$key])) {
              throw new ConfigurationException($key." must be a string.");
          }
          return $this;
      }
docs: Assert the property is string or is not set
  @param array $config Array being validated
  @param string|int $key Inspected array key
  @return $this
  @throws ConfigurationException On a failed assertation