delete_files() not working correctly in CI3

This call is not working correctly. I'd like to delete uploads/richb2-gmail-com/ and all its subdirs. This my structure
uploads/richb2-gmail-com/thumbnails

        $szBucket_name = str_replace(array("@", "."), "-", $_SESSION['userid']);
        $ouputDirectory = '/app/assets/uploads/'.$szBucket_name.'/';
        delete_files($ouputDirectory,TRUE);   << $outputDirectory is 

/app/assets/uploads/richb201-gmail-com/
But when this runs it only deletes the following dir and it's files. It skips deleting /richb201-gmail-com
/app/assets/uploads/richb201-gmail-com/thumbnails

proof that an old dog can learn new tricks