[racket] PLAI An inauspicious start

Trying out the first example in the book

#lang plai
(define (parse sexp)
  (cond
    [(number? sexp) (num sexp)]
    [(list? sexp)
     (case (first sexp)
       [(+) (add (parse (second sexp))
                 (parse (third sexp)))]
       [(-) (sub (parse (second sexp))
                 (parse (third sexp)))])]))

I get 

<unsaved editor>:4:21: expand: unbound identifier in module in: num

FWIW I have installed the patch on the books webpage and am running Racket 5.01 on Ubuntu.
 		 	   		  
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