code: protected function processDefault($url)
      {
          if ($url === null) {
              return $url;
          }
  
          $url = (string)$url;
  
          switch ($url) {
          case '404':
          case 'mm':
          case 'identicon':
          case 'monsterid':
          case 'wavatar':
          case 'retro':
              break;
          default:
              $valid = filter_var($url, FILTER_VALIDATE_URL);
              if (!$valid) {
                  throw new InvalidArgumentException('Invalid default avatar URL');
              }
              break;
          }
  
          return $url;
      }
docs: Verify and cast the default URL to use when no avatar image can be found.
  If none is set, the libravatar logo is returned.
  
  @param string $url Full URL to use OR one of the following:
  - "404" - give a "404 File not found" instead of an image
  - "mm"
  - "identicon"
  - "monsterid"
  - "wavatar"
  - "retro"
  
  @return string Default URL
  
  @throws InvalidArgumentException When an invalid URL is given