Apache2 w/ proxy balance and Rails: 404 error

I’m using proxy balancer to pass requests to a cluster of mongrels.
When I try and access a non-existant page I get back a page with a
text header, then the html of the correct 404 page. The browser
renders this page as plain text. If I bypass apache and hit the
mongrels directly on their ports I get the correctly rendered page.

Any suggestions?