TypeError: db.collection is not a function Firestore

TypeError: db.collection is not a function Firestore

I was presented this error in an attempt to import Firestore from the Firebase Admin SDK. Can’t find anything that helped, or if I imported it incorrectly

Code; index.js (main file)

var admin = require("firebase-admin");
var serviceAccount = require("./firebase/key.json");

admin.initializeApp({
  credential: admin.credential.cert(serviceAccount)
});

var db = admin.firestore()

Code; validate.js (import)

var db = require('../../../index')```