I would have told you how if I could understand what the error is. Which validator site are you checking and how are you checking it? I don't know if you are aware, but one of the things you should do while checking is to run it against local HTML (meaning, you need to run it after all the JS has executed on the page). If you have the WebDeveloper extension for Firefox, that gives you the option to validate local HTML.
I use the auto-generated CSS link on quite a few sites and it has never reported a validation error on that count.