无法读取admin.firestore.collection.doc.get.then.doc中未定义的属性“firstname”

我创建了这个firebase函数(oncreate user trigger)来返回新的用户数据并创建一个实时通知。

const functions = require('firebase-functions')
const admin = require('firebase-admin')

admin.initializeApp(functions.config().firebase)

const createNotification = (notification => {
    return admin.firestore().collection('notifications')
        .add(notification)
        .then(doc => console.log('notification added', doc))
})

exports.userJoined = functions.auth.user().onCreate(user => {
    return admin.firestore().collection('users').doc(user.uid).get().then(doc => {
        const newUser = doc.data()
        const notification = {
            content: 'Joined the app',
            user: `${newUser.firstName} ${newUser.lastName}`,
            time: admin.firestore.FieldValue.serverTimestamp()
        }

        return createNotification(notification)
    })
})

我第一次使用 firebase deploy 并创建一个新用户,触发器工作得很好。但是第二次和随后的尝试 Firebase Function logs 返回以下错误:

TypeError: Cannot read property 'firstName' of undefined
    at admin.firestore.collection.doc.get.then.doc (/user_code/index.js:28:29)
    at process._tickDomainCallback (internal/process/next_tick.js:135:7)