java.lang.IllegalStateException: Fatal Exception thrown on Scheduler.Worker thread.

[y2k]

java.lang.IllegalStateException: Fatal Exception thrown on Scheduler.Worker thread.
    at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:62)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:422)
    at java.util.concurrent.FutureTask.run(FutureTask.java:237)
    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:152)
    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:265)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
    at java.lang.Thread.run(Thread.java:841)
Caused by: java.lang.NoClassDefFoundError: java.util.concurrent.ConcurrentHashMap$KeySetView
    at y2k.joyreactor.common.PersistentMap.flush(PersistentMap.kt:25)
    at y2k.joyreactor.http.CookieStorage.grab(CookieStorage.kt:31)
    at y2k.joyreactor.http.HttpClient.getDocument(HttpClient.kt:80)
    at y2k.joyreactor.services.requests.UserNameRequest$request$1.invoke(UserNameRequest.kt:14)
    at y2k.joyreactor.services.requests.UserNameRequest$request$1.invoke(UserNameRequest.kt:10)
    at y2k.joyreactor.common.ObservableExtensionsKt$ioObservable$1$1.call(ObservableExtensions.kt:26)
    at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:55)
    ... 7 more