minifier not working

I'm trying to get the js- and cssminifier working. I keep getting this error message:

 Non-free jsminifier not available

It used to work in a earlier version of phalcon. does anyone know what this error massage is trieing to tell me?

I use the standard libs for minification:

use Phalcon\Assets\Filters\Cssmin;
use Phalcon\Assets\Filters\Jsmin;