Problem using %author% in permalinks

I’m trying to change my permalink structure from

/%category%/%postname%/

to

/%category%/%author%/%postname%/

but for some reason I’m getting a 404 at the new url.

It works fine with just the category and the postname, but when I add the author on there I get the error.

My site is at: http://www.bugsandcranks.com

Right now I’m just using category and postname since that’s what works.

Any ideas? Any help is much appreciated.

Thanks