Gzip outside of root directory?

When I enable the Gzip functions, I get error messages that the gss.gz file is outside the open base dir. I located the problem at line 61 and 62 of css_gzip.module. The $css_file is something like '/css/827498247923.css.gz'. However, that is outside the server root! I could fix the problem by adding $_SERVER['DOCUMENT_ROOT'] and $path in front of $css_file, like so:

          if (!file_exists($_SERVER['DOCUMENT_ROOT'] . $path . $css_file .'.gz')) {
            file_save_data(gzencode(file_get_contents(file_directory_path() . $css_file), 9), $_SERVER['DOCUMENT_ROOT'] . $path . $css_file .'.gz', FILE_EXISTS_REPLACE);
          }

What could be the real problem? Why does it want to store gzipped css files outside the root dir? The funny thing is, it does actually work with the errors.