[racket] on contract violation after adding 1471 strings to a BST

After studying chapter 14 of HtDP, I decided to try putting strings into a
BST. The code below works with a few strings. I have a file with 22064
strings --- one per line. Up until 1471 words, it works. With 1472 it
yields a contract violation.

> (length (read-words))
22064

> (create-bst-word-from-list (take (read-words) 1471))
#<node>

> (create-bst-word-from-list (take (read-words) 1472))
node-word: contract violation
  expected: node?
  given: #<void>
  context...:
   /home/dbastos/public_html/rkt/functions.rkt:974:0: create-bst-word
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list
   /home/dbastos/public_html/rkt/functions.rkt:987:0:
create-bst-word-from-list...

>

It seems node-word says I gave it a #<void>. Perhaps my list of words
contains a #<void>, but when I search for it, I do not find it.

> (define ls (read-words))
> (length ls)
22064
> (length (filter string? ls))
22064
> (filter void? ls)
()

So I'm perplexed.

(define-struct node (word left right))

(define (create-bst-word bst str)
  (cond
   ((false? bst)
    (make-node str false false))
   ((string<? str (node-word bst)) ;; insert at the left
    (make-node (node-word bst)
               (create-bst-word (node-left bst) str)
               (node-right bst)))
   ((string>? str (node-word bst)) ;; insert at the right
    (make-node (node-word bst)
               (node-left bst)
               (create-bst-word (node-right bst) str)))))

(define (create-bst-word-from-list ls)
  (cond
   ((empty? ls) false)
   (else
     (create-bst-word (create-bst-word-from-list (rest ls)) (first ls)))))

(define (read-lines in)
  (let* ((ln (read-line in)))
    (cond
     ((eof-object? ln) empty)
     (else (cons ln (read-lines in))))))

(define (read-words)
  (call-with-input-file "words" read-lines))

Thank you in advance for any information.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20140805/78dad2a9/attachment.html>