[racket] Holzweg-PLAI. Not doable in DrRacket 6.0.1?

Not sure how to proceed with PLAI per
http://cs.brown.edu/courses/cs173/2012/book/Introduction.html The code
seems not to run, e.g.,

#lang plai

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

actually wants to be

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

. . . at least DrRacket doesn't complain when I F5. But then I'm fresh out
of guesses with

(define (good? [ma : MisspelledAnimal]) : boolean
  (type-case MisspelledAnimal ma
    [caml (humps) (>= humps 2)]
    [yacc (height) (> height 2.1)]))

Please advise.

LB
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