Zip::add - Filename 2° parameter missing

API page: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Archiver%...

Enter a descriptive title (above) relating to public function Zip::add, then describe the problem you have found:

public function add($file_path,$filename=NULL) {
if($filename){
$this->zip
->addFile($file_path,$filename);
}else{
$this->zip
->addFile($file_path);
}

return $this;
}