Missing .aspx page not returning a 404

I have a website which has a generic error page "error.html" in the root. This is set for all errors in IIS 6.

If I type in domain.com/blah.html or domain.com/blah.js or indeed any NONE ASP.NET file extension then firebug tells me I get a 404 response.

If, however, I type in domain.com/blah.aspx then I get the error page but a 200 response.

Does anyone have any suggestions on how to get the 404 response correctly for missing aspx files?

Thanks