Why did adding a line of code to expire the header on a page break IIS?

Question

Possible Duplicate:
IIS6 + HttpModule: This operation requires IIS integrated pipeline mode

I added the following line's to a web page and it has crashed IIS....

Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetExpires(DateTime.UtcNow.AddYears(-1));
Response.Headers.Add("Cache-Control", "no-cache, no-store");

The error I am now getting when I browse to the page is:

Server Error in '/' Application.
This operation requires IIS integrated pipeline mode. 

Any ideas?