[plt-scheme] contract question

Finally I decided to try contracts. I tried:

#| require one or two numbers
   promise to return a number if two numbers are given
   promise void if one number is given |#

#lang scheme

(provide/contract
 (example
  (or/c
   (-> number? number? number?)
   (-> number? void?))))

(define (example a (b #f))
 (if b (+ a b) (void)))

This compiles, but it wont run:

Welcome to DrScheme, version 3.99.0.10-svn4feb2008 [3m].
Language: Module custom.
or/c: two arguments, (-> number? void?) and (-> number? number? number?), might both match #<procedure:example>

Why? What is wrong in my contract? May be I am wanting an impossible contract?
or/c repeats exactly what I want, but does not accept it.
I think I am missing something fundamental, but I can't find an answer in the docs.
Thanks, Jos Koot
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20080206/3ab82bec/attachment.html>