Erro tentando fazer um ator persistente exigir uma caixa de correio personalizada com RequerMessageQueue

Tento o seguinte:

class ClientActor (stockActor: ActorSelection, paymentsActor: ActorSelection) extends PersistentActor
  with AtLeastOnceDelivery with akka.actor.ActorLogging with RequiresMessageQueue[akka.custom.CustomMailbox]{
//actor implementation

}

Eu recebo o erro:

illegal inheritance;

     self-type Shopping.ClientActor does not conform to akka.dispatch.RequiresMessageQueue[akka.custom.CustomMailbox]'s selftype akka.dispatch.RequiresMessageQueue[akka.custom.CustomMailbox]
      with AtLeastOnceDelivery with akka.actor.ActorLogging with RequiresMessageQueue[akka.custom.CustomMailbox]{

O mesmo funciona com um ator não persistente:

    class PaymentsActor extends Actor with akka.actor.ActorLogging with RequiresMessageQueue[akka.custom.CustomMailbox] {
    // actor implementation
}

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