Hello people, I'm getting crazy with a small problem here, I keep getting an error and I cant seem to figure out why, the code is supposed to change the range of a list, so if we give it a list with values (1 2 3 4) and we want to change the range in 11 to fourteen the result would be (11 12 13 14) the problem is that the last function called scale-list will give back an error saying:

Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)

anybody has a clue why? I use aquamacs as an editor thanks in advance

;;finds minimum in a list
(defun minimum(list)
  (car  (sort list #'<)))
;;finds maximum in a list
(defun maximum(list)
  (car (sort list #'>)))
;;calculates the range of a list
(defun range(list)
  (- (maximum list) (minimum list)))

;;this codes scales a value from a list 
(defun scale-value(list low high n)
     (+ (/ (* (- (nth (- n 1) list) (minimum list)) (- high low)) (range list)) low))

;and this code is supposed to scale the whole list 
(defun scale-list(list low high n)
  (unless (= n 0)
   (cons (scale-value list low high n) (scale-list list low high (- n 1)))))

(scale-list '(0.1 0.3 0.5 0.9) 20 30 4)