为什么我得到TypeError('Router.use()

我有这样的index.js文件,

const express = require('express');
const app = express();

//Import Routes
const authRoute = require('./routes/auth');

//Route Middlewares
app.use('/api/user', authRoute);

app.listen(3000, () => console.log('Server Up and running'));

auth.js就像这样

const router = require('express').Router();

router.post('/register', (req, res) => {
    res.send('Register');
})
module.exports = router;

为什么我得到TypeError('Router.use()需要中间件功能,但是却得到了'+ gettype(fn))       ^

TypeError:Router.use()需要一个中间件功能,但有一个Object     在Function.use(C:\ Users \ xxx \ Desktop \ Websites \ Authentication \ node_modules \ express \ lib \ router \ index.js:458:13)     在功能。 (C:\ Users \ xxx \ Desktop \ Websites \ Authentication \ node_modules \ express \ lib \ application.js:220:21)