Error on Master Nodes

ES 2.3.1

On my Master Nodes I am seeing the below error:

[2016-06-02 08:45:11,438][INFO ][cluster.metadata         ] [MASTER_NODE_1] [cobalt-bro-2016.06.02] update_mapping [BRO]
[2016-06-02 08:54:55,933][DEBUG][action.admin.cluster.stats] [MASTER_NODE_1] failed to execute on node [tqihTxVCQx-NswEHjpEtqA]
RemoteTransportException[[WORKER_NODE_12][10.1.55.19:9300][cluster:monitor/stats[n]]]; nested: AlreadyClosedException[this IndexReader is closed];
Caused by: org.apache.lucene.store.AlreadyClosedException: this IndexReader is closed
        at org.apache.lucene.index.IndexReader.ensureOpen(IndexReader.java:274)
        at org.apache.lucene.index.CompositeReader.getContext(CompositeReader.java:101)
        at org.apache.lucene.index.CompositeReader.getContext(CompositeReader.java:55)
        at org.apache.lucene.index.IndexReader.leaves(IndexReader.java:438)
        at org.elasticsearch.search.suggest.completion.Completion090PostingsFormat.completionStats(Completion090PostingsFormat.java:330)
        at org.elasticsearch.index.shard.IndexShard.completionStats(IndexShard.java:765)
        at org.elasticsearch.action.admin.indices.stats.CommonStats.(CommonStats.java:164)
        at org.elasticsearch.action.admin.cluster.stats.TransportClusterStatsAction.nodeOperation(TransportClusterStatsAction.java:108)
        at org.elasticsearch.action.admin.cluster.stats.TransportClusterStatsAction.nodeOperation(TransportClusterStatsAction.java:54)
        at org.elasticsearch.action.support.nodes.TransportNodesAction.nodeOperation(TransportNodesAction.java:92)
        at org.elasticsearch.action.support.nodes.TransportNodesAction$NodeTransportHandler.messageReceived(TransportNodesAction.java:230)
        at org.elasticsearch.action.support.nodes.TransportNodesAction$NodeTransportHandler.messageReceived(TransportNodesAction.java:226)
        at org.elasticsearch.transport.RequestHandlerRegistry.processMessageReceived(RequestHandlerRegistry.java:75)
        at org.elasticsearch.transport.netty.MessageChannelHandler$RequestHandler.doRun(MessageChannelHandler.java:300)
        at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)

I did some googling but cannot see anything for elasticsearch and [this IndexReader cannot be used anymore as one of its child readers was closed].

This happens a lot and it is effecting indexing. Any help into why I am seeing this would be appreciated.

See: RemoteTransportException - AlreadyClosedException[this IndexReader is closed]

Thank you. I will try an update to ES 2.3.3