sent to error page even thought there is no error?

const express = require('express');
const app = express();
const port = 3000;
const bodyparser = require('body-parser');
const cookieparser = require('cookie-parser');

app.use(bodyparser.urlencoded({extended: false}));
app.use(cookieparser);

app.use((req, res, next)=>{
    const err = new Error('Uh noes!');
    next(err);
    err.status = 500;
});

app.set('view engine', 'pug');

app.listen(3000, ()=>{
    console.log(`app is listening on port ${port}`);
});

const mainroute = require('./routes');
app.use(mainroute);

app.use((err, req, res , next)=>{
    res.status(err.status);
    res.render('error', err);
});