ConcurrentModificationException on disconnect

This is reliably triggered when more than one bucket is open (works with just one):

CouchbaseCluster cluster = CouchbaseCluster.create();

Bucket bucket2 = cluster.openBucket("beer-sample");
Bucket bucket1 = cluster.openBucket("default");

cluster.disconnect();

Also, when the buckets are closed serially first it works.
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Exception in thread "main" java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
at java.util.HashMap$KeyIterator.next(HashMap.java:1453)
at rx.internal.operators.OnSubscribeFromIterable$IterableProducer.request(OnSubscribeFromIterable.java:96)
at rx.internal.operators.OperatorSubscribeOn$1$1$1$1.request(OperatorSubscribeOn.java:88)
at rx.Subscriber.setProducer(Subscriber.java:143)
at rx.Subscriber.setProducer(Subscriber.java:137)
at rx.internal.operators.OperatorSubscribeOn$1$1$1.setProducer(OperatorSubscribeOn.java:81)
at rx.internal.operators.OnSubscribeFromIterable.call(OnSubscribeFromIterable.java:47)
at rx.internal.operators.OnSubscribeFromIterable.call(OnSubscribeFromIterable.java:33)
at rx.Observable.unsafeSubscribe(Observable.java:7658)
at rx.internal.operators.OperatorSubscribeOn$1$1.call(OperatorSubscribeOn.java:62)
at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:43)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)