code: 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;
      }
docs: Assert the value is array of strings 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