help with caching

I'm using lighttpd serving mongrel backends. I have the following
rewrites in my lighty conf file:
url.rewrite += ( "^/$" => "/cache/index.html" )
url.rewrite += ( "^([^.]+)$" => "/cache/$1.html" )

If a page has already been cached (and exists in public/cache), it
gets served properly. However, if the page hasn't
yet been cached, mongrel is looking for the cached page, can't find it
and throws a Routing Error.

What do I need to do to correct this?