Field value equal "Unknown" get error

Elastic Stack Elasticsearch
1

I get the error message as below:

"error"=>{"type"=>"mapper_parsing_exception", "reason"=>"failed to parse [details-RequestData-MessageTag]", "caused_by"=>{"type"=>"illegal_argument_exception", "reason"=>"For input string: "Unknown""}}

And in my json

{"details-RequestData-MessageTag":"Unknown"}

Is it a bug or some rules that not allow to use "Unknown"?

2

What exactly are you doing?

3

I am sending data to logstash.
But I got this error when it write to elasticsearch

4

Ok but I still have no idea on what exactly you are doing.