IIS sets http header Expires to -1, how do I disable this?

Question

I'm using an IIS website with a Apache HTTP reverse proxy (mod_proxy with ProxyRequests Off). I want to control the expires headers in apache (don't ask), but IIS is setting the header Expires: -1.

This causes mod_expires to ignore the request since it already has a value.

I tried the solutions in http://stackoverflow.com/questions/7930140/overwrite-cache-headers-with-mod-expires, but it doesn't work (the early trick doesn't work).

How do I disable IIS to specify Expires: -1 ?

OR

How do I force mod_expires to override it (since I'm using apache as a reverse proxy, I should be able to do this!)