Core async question

sample.clj

;; Время от времени появляется ошибка #object[clojure.core.async.impl.channels.ManyToManyChannel
;; Почему? По идее `random-add` никак не влияет на заполнение канала, только на таймаут?

(ns async-tea-party.core
  (:require [clojure.core.async :as async]))

(def google-tea-service-chan (async/chan 10))
(def yahoo-tea-service-chan (async/chan 10))
(def result-chan (async/chan 10))


(defn random-add []
  (reduce + (conj [] (repeat 1 (rand-int 100000)))))

(defn request-google-tea-service []
  (async/go
    (random-add)
    (async/>! google-tea-service-chan
              "tea compliments of google")))

(defn request-yahoo-tea-service []
  (async/go
    (random-add)
    (async/>! yahoo-tea-service-chan
              "tea compliments of yahoo")))

(defn request-tea []
  (request-google-tea-service)
  (request-yahoo-tea-service)
   (async/go (let [[v] (async/alts!
                        [google-tea-service-chan
                         yahoo-tea-service-chan])]
               (async/>! result-chan v))))

(defn -main [& args]
  (println "Requested tea!")
  (request-tea)
  (println (async/<!! result-chan)))