Lambda the Ultimate

Hi guys, I am a freshman student in Kenya. After I stumbled on MIT's ocw website, I got interested in Scheme and after a few while on this LtU site, I felt like starting with DrScheme is the best way since its tailoured to the biginners. I am very new in Programming and we just don't have great teachers who have command of the subject here in Kenya.
Well after installing and running DrScheme, I encountered a problem which I will be very greatful to anyone of you who can help.
I difined this procedure: (define (area-of-disk r)
(* 3.14 (* r r)))
which worked when I ran like this (area-of-disk 5)

then I defined this: (define (area-of-ring outer inner)
(- (area-of-disk outer)
(area-of-disk inner)))
However upon running this procedure I got this message; area-of-disk: name is not defined, not an argument, and not a primitive name.
and this on the tracing window: No tracing results are available, yet. (Make sure that your language supports tracing and that tracing is enabled.)
Can anyone help?