Membership breaking minify with W3 Total Cache

Hi

I'm using Membership with W3 Total Cache. I'm using W3 to minify my CSS files. The minified file that's created (in wp-content/cache)is being protected by Membership and causing the layout to break.

If I browse directly to the minified css file, I see the No Access PNG.If I deactivate Membership the minified file is viewable and the layout is fine.

I've not specified that that path (wp-content/cache) is protected via Membership.

Any ideas?

Thanks
Pete