CouchbaseException: INVALID_ARGUMENTS on bulkInsert

Hi,
I am doing BulkInsert, I am stuck due to this Exception and not getting why this Exception occurred.

Inserting with batch of 2000 with 10 threads
using couchbase : Community Edition 6.0.0 build 1693
couchbase-client.version: 2.6.0

@Override
public <T> List<Result> bulkInsert(List<T> objects) {
    List<RawJsonDocument> jsonDocuments = CouchbaseUtils.createJsonDocumentsAsync(objects);
    // Insert them in one batch, waiting until the last one is done.
    List<RawJsonDocument> responses = Observable
        .from(jsonDocuments)
        .observeOn(scheduler)
        .flatMap(new Func1<RawJsonDocument, Observable<RawJsonDocument>>() {
            @Override
            public Observable<RawJsonDocument> call(final RawJsonDocument docToInsert) {
                return bucket.async().insert(docToInsert);
            }
        })
        .toList()
        .last()
        .toBlocking()
        .single();
    return responses.stream().map(response -> Result.builder().docId(response.id()).rev(String.valueOf(response.cas())).build())
        .collect(Collectors.toList());
}

Getting following exception. list of objects contain different characters not only english like Thai, Chinese, Arabic, Italian

    com.couchbase.client.core.CouchbaseException: INVALID_ARGUMENTS
	at com.couchbase.client.java.bucket.api.Mutate$1$1.call(Mutate.java:161) ~[java-client-2.6.0.jar:?]
	at com.couchbase.client.java.bucket.api.Mutate$1$1.call(Mutate.java:132) ~[java-client-2.6.0.jar:?]
	at rx.internal.operators.OnSubscribeMap$MapSubscriber.onNext(OnSubscribeMap.java:69) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.observers.Subscribers$5.onNext(Subscribers.java:235) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.internal.operators.OnSubscribeDoOnEach$DoOnEachSubscriber.onNext(OnSubscribeDoOnEach.java:101) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.internal.producers.SingleProducer.request(SingleProducer.java:65) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.internal.producers.ProducerArbiter.setProducer(ProducerArbiter.java:126) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.internal.operators.OnSubscribeTimeoutTimedWithFallback$TimeoutMainSubscriber.setProducer(OnSubscribeTimeoutTimedWithFallback.java:155) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.Subscriber.setProducer(Subscriber.java:205) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.internal.operators.OnSubscribeMap$MapSubscriber.setProducer(OnSubscribeMap.java:102) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.Subscriber.setProducer(Subscriber.java:205) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.Subscriber.setProducer(Subscriber.java:205) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.subjects.AsyncSubject.onCompleted(AsyncSubject.java:103) ~[rxjava-1.3.7.jar:1.3.7]
	at com.couchbase.client.core.endpoint.AbstractGenericHandler.completeResponse(AbstractGenericHandler.java:508) ~[core-io-1.6.0.jar:?]
	at com.couchbase.client.core.endpoint.AbstractGenericHandler.access$000(AbstractGenericHandler.java:86) ~[core-io-1.6.0.jar:?]
	at com.couchbase.client.core.endpoint.AbstractGenericHandler$1.call(AbstractGenericHandler.java:526) ~[core-io-1.6.0.jar:?]
	at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:55) ~[rxjava-1.3.7.jar:1.3.7]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_151]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_151]
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180) ~[?:1.8.0_151]
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293) ~[?:1.8.0_151]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_151]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_151]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_151]
Caused by: rx.exceptions.OnErrorThrowable$OnNextValue: OnError while emitting onNext value: com.couchbase.client.core.message.kv.InsertResponse.class
	at rx.exceptions.OnErrorThrowable.addValueAsLastCause(OnErrorThrowable.java:118) ~[rxjava-1.3.7.jar:1.3.7]
	at rx.internal.operators.OnSubscribeMap$MapSubscriber.onNext(OnSubscribeMap.java:73) ~[rxjava-1.3.7.jar:1.3.7]
	... 21 more