@font-face url after minify css assets

Hi. As you can see in the code below

$cssCollection = $this->assets->collection('css_src');
        $cssCollection->setTargetPath('final.css');
        $cssCollection->setTargetUri('final.css');
        $cssCollection->addCss('https://fonts.googleapis.com/css?family=Open+Sans:400,600', false);
        $cssCollection->addCss('ilya-theme/backend/assets/bower_components/bootstrap/css/bootstrap.min.css');
        $cssCollection->addCss('ilya-theme/backend/assets/icon/icofont/css/icofont.css');
        $cssCollection->join(true);
        $cssCollection->addFilter(new Cssmin());

after making final.css , the font-face src does not refer to its orginal path and do not work on the site output the fonts.

final.css

@font-face {
    font-family: 'icofont';
    src: url('../fonts/icofont.eot?v=1.0.0-beta');
    src: url('../fonts/icofont.eot?v=1.0.0-beta#iefix')

Orginal path icofont.css and fonts

ilya-theme
                backend
                            assets
                                        icon
                                                icofont
                                                            css
                                                                    icofont.css
                                                            fonts
                                                                    icofont.eot

What is the problem with my code?