Rails - Oct 2005 - caches_page creates empty file for non-exist page

I''m trying caches_page to cache an action(img file from db).  If the
file doesn''t exist, I just do following in my code:

render(:nothing => true, :status => 404)

Rails then creates an cache file with zero byte, and I can see the
status code as 404 in Apache access_log.

However the next time when try to access the same url, the access_log
shows status code of 200 instead of 404.  Certainly this is due to the
existence of empty cache file.

Is it a feature(?) or bug?  How to avoid/work around it?

Thanks,
Kevin