Tiny Forsight, trying to figure out error

tinyforsight.ss

#lang racket
(define *prompt* "> ")

(define *dictionary* '(("+" add-f)))

(define (push item stack)
  (cons item stack))

(define (pop stack)
  (rest stack))

(define (pop-2 stack)
  (rest (rest stack)))

(define (mk-binary op)
  (lambda (stack)
    (push 
     (op (second stack) (first stack)) 
     (pop-2 stack))))

(define add-f (mk-binary +))

(define (tokenize str)
  (define (splitter str i last stop sep)
    (cond
      ((eq? i stop)
        (cons (substring str last stop) '()))
      ((equal? (string-ref str i) sep)
        (cons (substring str last i) (splitter str (+ i 1) (+ i 1) stop sep)))
      (else
        (splitter str (+ i 1) last stop sep))))
  (splitter str 0 0 (string-length str) #\space))

(define (dict-get key dict)
  (cond
    ((null? dict)
      #f)
    ((equal? key (first (first dict)))
      (first (rest (first dict))))
    (else
      (dict-get key (rest dict)))))

(define (interpret-f input d-stack)
  (cond
    ((null? input)
     d-stack)
    ((string->number (first input))
     (interpret-f (rest input) (push (string->number (first input)) d-stack)))
    (else
     (let ((fun-f (dict-get (first input) *dictionary*)))      
       (interpret-f (rest input) ((eval fun-f) d-stack))))))

(define (repl-f d-stack)
  (display *prompt*)
  (let* ((input (read-line))
         (new-stack (interpret-f (tokenize input) d-stack)))
       (repl-f new-stack)))

(repl-f '())