语法错误:意外的保留字,用于等待循环

我有一个这样的功能:

exports.deleteUser = functions.https.onCall(async(data, context) => {
    let id = context.auth.uid;
    console.log('Delete user: ' + id);

    //delete from algolia
    usersIndex.deleteObject(id);
    console.log(id + 'Deleted from algolia');

    //delete user following
    await admin.firestore().collection('users').doc(id).collection('Following').get()
      .then(async(snapshot) => {
        for await (const document of snapshot.docs) {
          await admin.firestore().collection('users').doc(document.documentId)
            .update({
              'NumberOfFollowers': FieldValue.increment(-1)
            });
          await admin.firestore().collection('users').doc(document.documentId).collection('Followers')
            .doc(id).delete();
        }
        return console.log('Following of ' + id + ' deleted');
      });
...

但是当我尝试将其部署到firebase函数时,出现以下错误:

!  functions[deleteUser(us-central1)]: Deployment error.
Function failed on loading user code. Error message: Code in file index.js can't be loaded.
Is there a syntax error in your code?
Detailed stack trace: /srv/index.js:47
            for await (const document of snapshot.docs) {
                ^^^^^

SyntaxError: Unexpected reserved word
    at createScript (vm.js:80:10)
    at Object.runInThisContext (vm.js:139:10)
    at Module._compile (module.js:617:28)
    at Object.Module._extensions..js (module.js:664:10)
    at Module.load (module.js:566:32)
    at tryModuleLoad (module.js:506:12)
    at Function.Module._load (module.js:498:3)
    at Module.require (module.js:597:17)
    at require (internal/module.js:11:18)
    at getUserFunction (/worker/worker.js:439:24)

Found this: https://github.com/nodejs/node/issues/21617 but I did set my ForLoop in an async function...