Why is the Content Header 'application/javascript' causing a 500 Error?

Question

I have a script that works fine on my test server (using IIS6). The script processes an ajax request and sends a response with the following line:

header( 'application/javascript' ); 

But on my live server, this line crashes the page and causes a 500 error.

Do I need to allow PHP to send different MIME types in IIS7? If so, how do I do this? I can't find any way on the interface.