null pointer exception

a.clj

; no more null pointer :)
(use 'clojure.set)

(def tens (range 1 10))
(def ones (range 0 10))

(defn combine [a b] (+ (* 10 a) b))
(defn uniq [a b n] (if (= a n) b a))
(defn safed [a b] (if (zero? b) 0 (/ a b)))
(def answers
  (for [a tens b ones c tens d ones
    :when (let [
      inter (intersection (set [a b]) (set [c d]))
      inter-int (first inter)
      ab (combine a b)
      cd (combine c d) ]
      (and
        (= 1 (count inter))
        (< (safed ab cd) 1)
        (not (= 0 b d))
        (= (safed ab cd) (safed (uniq a b inter-int) (uniq c d inter-int)))
        ))]
    [(combine a b) (combine c d)]))

(prn answers)
(prn (map #(/ (first %) (last %)) answers))
(prn (reduce * (map #(/ (first %) (last %)) answers)))