读取uni-id-users的奇怪问题

读uni-id-users表，不是说要限制字段，不能读password吗？ 但是我这里是不限制字段反而能读取，限制了字段就报错。

代码就是最简单测试代码：
exports.main = async (event, context) => {
const res = await uniCloud.database().collection("uni-id-users").get()
console.log(res);
return res
};

1.先不限制字段，一切正常，正常输出结果。

1. 限制字段，改为 const res = await uniCloud.database().collection("uni-id-users").field("username").get()，
   加了一个field，结果报错

12:29:53.324 [本地调试]TypeError: Cannot assign to read only property '0' of string 'username'
12:29:53.337 [本地调试] at mt.field (D:\work\uniapp\HBuilderX\plugins\unicloud\aliyun\@dcloudio\serverless\lib\aliyun\uni-cloud.js:1:39238)
12:29:53.352 [本地调试] at Object.exports.main (D:\work\uniapp\projects\customer_client\uniCloud-aliyun\cloudfunctions\getData\index.js:6:70)
12:29:53.367 [本地调试] at Q (D:\work\uniapp\HBuilderX\plugins\unicloud\aliyun\serve.js:1:11248)
12:29:53.387 [本地调试] at se (D:\work\uniapp\HBuilderX\plugins\unicloud\aliyun\serve.js:1:11763)
12:29:53.406 [本地调试] at userAgent (D:\work\uniapp\HBuilderX\plugins\unicloud\aliyun\serve.js:1:12731)
12:29:53.423 [本地调试] at module.exports (D:\work\uniapp\HBuilderX\plugins\unicloud\aliyun\serve.js:1:12737)
12:29:53.441 [本地调试] at module.exports.exec (D:\work\uniapp\HBuilderX\plugins\unicloud\server\controller\cloudfunctions.js:1:2446)
12:29:53.458 [本地调试] at processTicksAndRejections (internal/process/task_queues.js:97:5)
12:29:53.486 [本地调试] at async Server.<anonymous> (D:\work\uniapp\HBuilderX\plugins\unicloud\server\index.js:1:1879)