Weird URL Rewriting error...

[Jeremy]

I have written my own URL rewriter, and it was working fine until i moved
the re-writing code from App_BeginRequest to App_AuthorizeRequest.

Now I get a 404 (file not found) error whenever the request doesn't have a
query string. If I add a query string, it works fine. The really weird thing
is that when I turn tracing on, the 404 goes away! This is driving me
crazy. Here are some sample URLs:

(tracing enabled = False)
Rewritten URL: /Default.aspx
Physical URL: /category.aspx?id=default
Response: 404

Rewritten URL: /Default.aspx?foo=bar
Physical URL: /category.aspx?id=default&foo=bar
Response: OK

Any thoughts?

Thanks,
Jeremy