Sub document update for a nested array document

Java SDK
2

If i use the mutateIn option, it is treating the fragment as a string, and replaces the structure with quoted escaped String.
If I send the JsonObject as the fragment to be updated,

I get “BinaryResponse{bucket=‘bucketName’, status=INVALID_ARGUMENTS (4), request=SubDictUpsertRequest{observable=rx.subjects.AsyncSubject@147bcbe1, bucket=‘bucketName’}, content=EmptyByteBufBE}”