Assetic + YUI Compressor in symfony 2: is this a bug?

I have tested the YUI compressor in the command line (on windows) and it seems to work just fine.

But the .css created by assetic is not compressed, and comes with this message on the top (inside the .css!):

/*
[exception] 500 | Internal Server Error | RuntimeException
[message] 
[1] RuntimeException: 
            at n/a
                in E:\websites\symfony2\public_html\Symfony\vendor\assetic\src\Assetic\Filter\Yui\BaseCompressorFilter.php line 81

            at Assetic\Filter\Yui\BaseCompressorFilter->compress('

Is this a configuration problem? Or a bug in assetic?

Here's the code I used inside my twig template:

{% stylesheets '@CompanyBundlenameBundle/Resources/public/css/style.css' filter='yui_css' %}
<link rel="stylesheet" type="text/css" media="screen" href="{{ asset_url }}" />
{% endstylesheets %}