Why can't I recur here?

calculate.clj

(defn calculate
  "A list of coin-values from coins, which sum to target"
  [coins target]
  (let [coin (max-coin coins target)]
    (cond
      (not coin)      (js-alert "No solution")
      (= coin target) (list coin)
      (< coin target) (cons coin (recur coins (- target coin))))))

recursive.clj

(defn calculate
  "Make the value of target out of values in the list of coins"
  ([coins target]
    (reverse (calculate coins target (list))))
  ([coins target so-far]
    (let [coin (max-coin coins target)]
      (cond
        (not coin)      (js-alert "No solution")
        (= coin target) (cons coin so-far)
        (< coin target) (recur coins (- target coin) (cons coin so-far))))))

AssertionError Assert failed: Can't recur here