[racket] PLAI question

Hi there,
I had a look at the PLAI book and tried out one of the first examples:

#lang plai


(define-type MisspelledAnimal
  [caml (humps : number)]
  [yacc (height : number)])

(caml 2)
(yacc 3.4)

However, I get:
plaitest.rkt:5:17: define-type: unexpected term
  at: number
  in: (define-type MisspelledAnimal (caml (humps : number)) (yacc
(height : number))) context...:
   /home/manfred/racket/lib/racket/collects/syntax/parse/private/runtime-report.rkt:31:0
   /home/manfred/racket/lib/racket/collects/racket/private/modbeg.rkt:46:4
   standard-module-name-resolver


I have no glue what's going wrong as I thought that language plai
supports the stuff. Must I define the number type?


-- 
Thanks,
Manfred