Mysql.JSON.为什么我得到-错误代码：1064

简单表格：

CREATE TABLE `LocalTest` (
  `mainJson` json NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

尝试插入json值：

insert into `LocalTest` (mainJson) values ('{ "key": "value" }') 

结果错误：

Error Code: 1064. You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'Error Code: 3140. Invalid JSON text: "The document root must not be followed by ' at line 3

这个最简单的脚本怎么了？