Non-free jsminifier not available

Hi all. I get error when use Assets Management

My code

$this->assets
                ->collection('js')
                    ->addJs('js/jquery-2.1.3.min.js')
                    ->addJs('js/bootstrap.min.js')
                    ->join(true)
                    ->setTargetPath('compile.js')
                    ->setTargetUri('js/compile.js')
                    ->addFilter(new \Phalcon\Assets\Filters\Jsmin());

<b>Fatal error</b>:  Uncaught exception 'Phalcon\Assets\Exception' with message 'Non-free jsminifier not available'

license issues or what that ?