在我之前的代码中，使用了RoundRobinRouter（在akka版本2.11-2.3.10中）。
val workerRouter = context.actorOf(Props(new DistanceCalculatingWork(frag)).withRouter(RoundRobinRoutor(groups)),name="workerRouter")
当我使用新版本的akka（v2.54）时，代码是由
加入的val workerRouter = context.actorOf(
Props(new DistanceCalculatingWork(frag)).withRouter(RoundRobinPool(groups)), name = "workerRouter")
或
val workerRouter = context.actorOf(RoundRobinPool(groups).props(Props(new DistanceCalculatingWork(frag))), name = "workerRouter")
但是演员无法运行，并且INFO报道：
[INFO] [02/28/2018 17:42:40.202] [CalculateDistances-akka.actor.default-dispatcher-7] [akka://CalculateDistances/user/DistanceCalculatingActor_Master] Message [com.panyl.protax.distance.DistanceActorCounter] from Actor[akka://CalculateDistances/user/DistanceCalculatingActor_Master/workerRouter/$h#2021427677] to Actor[akka://CalculateDistances/user/DistanceCalculatingActor_Master#-384110236] was not delivered. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.
问题是什么？以及如何解决它？