save Record got error

[yuchangfu]

I use this code to save a Record

HashMap<String ,Object> commentData = new HashMap<>();
                commentData.put("title","haha comment");
                commentData.put("data","world is bad");
                Record aComment = new Record("Comment",commentData);
                skygear.getPublicDatabase().save(aComment, new RecordSaveResponseHandler() {
                    @Override
                    public void onSaveSuccess(Record[] records) {
                        Log.i("MyApplication", "Record saved");
                    }

                    @Override
                    public void onPartiallySaveSuccess(Map<String, Record> successRecords, Map<String, Error> errors) {
                        Log.i("MyApplication", "Some records are failed to save");
                    }
                    @Override
                    public void onSaveFail(Error error) {
                        Log.w("MyApplication", "Failed to save: " + error.getMessage(), error);
                    }
                });

The request body like this:
{
"database_id": "_public",
"api_key": "123456789",
"access_token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJqdGkiOiIyNjc1N2UyZC01Yzg3LTQ4YTQtYjY2Ni05MmY1YWRlMjQ1MjAiLCJpYXQiOjE0OTc1ODA5MzMsImlzcyI6ImVhc3lib29rIiwic3ViIjoiMWNjOGViYWQtZGYwMy00MjAxLThkMWQtMmQ1YmYzOWYwNDQ4In0.BuD5x13I5TLQcdUz5CRAzscaMyWgnx5BnifFNcrq0Yc",
"action": "record:save",
"records": [{
"data": "world is bad",
"title": "haha comment",
"_id": "Comment/84004521-76ca-402e-a050-1ffd13876f32",
"_created_at": "2017-06-16T02:42:31.898Z",
"_updated_at": "2017-06-16T02:42:31.898Z",
"_access": [{
"level": "read",
"public": true
}]
}]
}

and get this error
{
"error": {
"name": "BadRequest",
"code": 107,
"message": "fails to decode the request payload"
}
}

What cause this problem and how to fix it