A foreign character é causes statusCode 400

[gezichenshan]

I sent POST http request with this body to "/lists/{list_id} | Batch sub/unsub list members":

{
"members":
  [{"email_address":"testtest@qq.com",
  "status":"subscribed",
  "merge_fields":{"FNAME":"César Octavio"}
  }],
"update_existing":true
}

and got

{
  "type":"http://developer.mailchimp.com/documentation/mailchimp/guides/error-glossary/",
  "title":"Invalid Resource",
  "status":400,
  "detail":"The resource submitted could not be validated. For field-specific details, see the 'errors' array.",
  "instance":"bae54115-1fd9-4d46-b203-775f0fdb063e",
  "errors":[{"field":"","message":"Schema describes object, NULL found instead"}]
}

I think it is this Spanish character é in "César Octavio" causes the issue. Because when I move it out, it works properly.

How can I fix it ? I worked a whole day trying to get it done but failed.