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

```json
{"details-RequestData-MessageTag":"Unknown"}
```
Is it a bug or some rules that not allow to use "Unknown"?

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