IIS7 Overrides customErrors when setting Response.StatusCode?

Having a weird problem here. Everybody knows that if you use web.config’s customErrors section to make a custom error page, that you should set your Response.StatusCode to whatever is appropriate. For example, if I make a custom 404 page and name it 404.aspx, I could put <% Response.StatusCode = 404 %> in the contents in order to make it have a true 404 status header.