Could not index event to Elasticsearch due to opType: [-1]

Hi all,

We encountered a transient issue with the [WARN] said:

Could not index event to Elasticsearch

From the log, the response is like

:response => {"create"=>{"_index"=>"zzzzzzzzzz"},
"_type"=>"_doc",
"_id"=>"xxxxxxxxx",
"status"=>"400",
"error"=>{"type"=>"illegal_argument_exception",
                  "reason"=>"Unknown opType: [-1]"}

This issue lasted 10+ seconds with 1000+ hits and disappeared by itself without any intervention.

I wonder what could be the possible cause? Another weird thing is I did not see any exception IllegalArgumentException from master node log.

Also, checked the index zzzzzzzzzz _stats, the failed index number is 0, but I think it should not.

Thanks!