Learning mount - getting an error

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Hello All,

I am learning mount library in Clojure GitHub - tolitius/mount: managing Clojure and ClojureScript app state since (reset) . This is my code:

(ns mount-example.foo
  (:require [mount.core :refer [defstate start stop]]))
;;=> nil

(defstate a
  :start 42
  :stop (println "Configuration cleared."))
;;=> #'mount-example.foo/a

(start)
;;=> Execution error (ClassCastException) at mount-example.core/eval491$fn (core.clj:20).
;;   class clojure.lang.PersistentArrayMap cannot be cast to class java.util.concurrent.Future (clojure.lang.PersistentArrayMap is in unnamed module of loader 'app'; java.util.concurrent.Future is in module java.base of loader 'bootstrap')
;;   

(stop)

I am not sure why I am getting the error above. Would be great if someone throws light upon it.

Thanks in advance.