I'm failing to append to list for some reason

Programming Common Lisp
1

I’m doing the Allergies and have this code:

(setq allergens '(("eggs" . 1)
                  ("peanuts" . 2)
                  ("shellfish" . 4)
                  ("strawberries" . 8)
                  ("tomatoes" . 16)
                  ("chocolate" . 32)
                  ("pollen" . 64)
                  ("cats" . 128)))

(defun allergic-to-p (score allergen)
  "Returns true if given allergy score includes given allergen."
  (when (>
         (logand
          score
          (or (cdr (assoc allergen allergens :test #'equal)) 0))
         0)
    t))

(defun list (score)
  "Returns a list of allergens for a given allergy score."
  (reduce #'(lambda (acc allergen)
              (if (allergic-to-p score (car allergen))
                  (cons (car allergen) acc)
                  acc))
          allergens
          :initial-value ()))

This produces the correct result but in reverse. So I want to change list to append to the list instead of prepending. To do this I tried to change (cons (car allergen) acc) to (append acc (list (car allergen))). But this gives me an error:

The value
  "eggs"
is not of type
  INTEGER
   [Condition of type TYPE-ERROR]

What am I missing?