我在子查询中有问题返回了超过1行的mysql错误

Question

执行子查询时出现错误。以下是我的代码，并且还附有错误图片：-

SELECT (
    SELECT SUM(`amount`) 
        FROM `test` 
    GROUP BY created_by
) + (
  SELECT SUM(`amount1`) 
        FROM `test1` 
    GROUP BY created_by
) AS `t`