[plt-scheme] top-level error

Hi,

    I am really confused that why DrSchme complains that:

BUG  reference to undefined identifier: make-stack

for the code below when i click "Run"


(define aim-cell-stk (make-stack))

(define make-stack
  (lambda ()
    (let ((ls '()))
      (lambda (msg . entries)
        (cond
          ((eqv? msg 'empty?)
           (null? ls))
          ((eqv? msg 'push!)
           (set! ls (cons (car entries) ls)))
          ((eqv? msg 'top)
           (car ls))
          ((eqv? msg 'pop!)
           (set! ls (cdr ls)))
          (else
           "Oops, invalid message!"))))))


Both are in the top-level environment, why the complaint?