When running snapshot, BufferedChecksumIndexInput cannot seek backwards

[garbagefire]

Hello,

We are running Elasticsearch 2.3.5.

About a month ago and a half ago, we optimized all of our indexes. E.g.:

POST /w-indexname-2016.07.21_v1.4/_optimize?max_num_segments=1

Since then, when we run our daily snapshop, we get this log entry for many of our indexes and shards:

[w-indexname-2016.07.21_v1.4][[w-indexname-2016.07.21_v1.4][2]] IndexShardSnapshotFailedException[class org.apache.lucene.store.BufferedChecksumIndexInput cannot seek backwards (pos=-16 getFilePointer()=0)]; nested: IllegalStateException[class org.apache.lucene.store.BufferedChecksumIndexInput cannot seek backwards (pos=-16 getFilePointer()=0)];
	at org.elasticsearch.index.snapshots.blobstore.BlobStoreIndexShardRepository.snapshot(BlobStoreIndexShardRepository.java:201)
	at org.elasticsearch.snapshots.SnapshotShardsService.snapshot(SnapshotShardsService.java:340)
	at org.elasticsearch.snapshots.SnapshotShardsService.access$200(SnapshotShardsService.java:76)
	at org.elasticsearch.snapshots.SnapshotShardsService$1.doRun(SnapshotShardsService.java:296)
	at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalStateException: class org.apache.lucene.store.BufferedChecksumIndexInput cannot seek backwards (pos=-16 getFilePointer()=0)
	at org.apache.lucene.store.ChecksumIndexInput.seek(ChecksumIndexInput.java:50)
	at org.apache.lucene.codecs.CodecUtil.checksumEntireFile(CodecUtil.java:450)
	at org.elasticsearch.repositories.blobstore.ChecksumBlobStoreFormat.readBlob(ChecksumBlobStoreFormat.java:97)
	at org.elasticsearch.index.snapshots.blobstore.BlobStoreIndexShardRepository$Context.buildBlobStoreIndexShardSnapshots(BlobStoreIndexShardRepository.java:481)
	at org.elasticsearch.index.snapshots.blobstore.BlobStoreIndexShardRepository$SnapshotContext.snapshot(BlobStoreIndexShardRepository.java:537)
	at org.elasticsearch.index.snapshots.blobstore.BlobStoreIndexShardRepository.snapshot(BlobStoreIndexShardRepository.java:191)
	... 7 more

Any thoughts?