Why is my instrument not working?

cashflows.clj

(ns cashflows
  (:require [clojure.spec.alpha :as s]
            [clojure.spec.test.alpha :as stest]))

(s/def ::date (s/inst-in #inst "1990" #inst "2050"))
(s/def ::amount number?)
(s/def ::cashflow (s/cat :date ::date :amount ::amount))

(s/exercise ::cashflow 5)

(defn date-from [cashflow]
  (nth cashflow 0))

(defn amount-from [cashflow]
  (nth cashflow 1))

(defn net-cashflow
  "Returns the net cashflow amount of a sequence of cashflows"
  [cashflows]
  (reduce #(+ %1 (amount-from %2)) 0 cashflows))

(s/fdef net-cashflow
  :args (s/coll-of ::cashflow)
  :ret number?)

(def example [[#inst "2020-01" -200]
               [#inst "2020-03" -200]
               [#inst "2020-07" -300]
               [#inst "2020-10" -300]
               [#inst "2021-08" 150]
               [#inst "2021-09" 150]
               [#inst "2021-11" 150]
               [#inst "2021-12" 50]
               [#inst "2022-06" 500]
               [#inst "2022-08" 120]])

(s/explain-str (s/coll-of ::cashflow) example) ;; true
(net-cashflow example) ;; instrument complains