How to use multiple filters in assets

I use the following code to process styles:

    $this->assets
        ->collection('css')
        ->setTargetPath('css/index.css')
        ->setTargetUri('css/index.css')
        ->addCss('../app/assets/css/style.scss')
        ->addFilter(new ScssFilter())
        ->addFilter(new \Phalcon\Assets\Filters\Cssmin());

At first it should pass the contents of style.scss through scss library to generate css and then compact this css with cssmin. But the result I get is a file, that contains both the generated css and minified scss which is wrong.

How can I tell the assets manager that it should pass the styles through both filters sequentially and return only the minified css?

P.S. Asset Manager has an addCss method which 4th argument is an array $attributes. And there is absolutely no description of it's purpose and format.