IIS7 & 404?

I'm guessing this must be a setting in IIS, but I have several sites running with custom 404 pages on IIS6 and all I did was put the nodeID in the <error404>1264</error404> section in the umbracoSettings.config

But I have done this on a new site in IIS7 but it doesn't work, I get the default IIS 404 page?  Am I missing something?