当我从firebase配置对象调用firestore时.

它说

firebase2.default.firestore is not a function

这是代码

import firebase from 'firebase'

const config = {
  apiKey: "AIzaSxxxxxxqpUeqKXI",authDomain: "payxxxxxa1.firebaseapp.com",databaseURL: "https://payxxxxxxxa1.firebaseio.com",projectId: "payxxxxxxja1",storageBucket: "payxxxxxa1.appspot.com",messagingSenderId: "281xxxxxxxx576"
}

firebase.initializeApp(config)

export const fire = firebase
export const ref = firebase.database().ref()
export const firebaseAuth = firebase.auth
export const messaging = firebase.messaging();
export const db = firebase.firestore();

Firebase版本是4.5.0

npm list firebase > firebase@4.5.0

我做错了吗？