My /cards route is not working!

This is from the Express Basics course, "Parameters, Query Strings, and Modularizing Routes" part, the first video "Modular Routes".

All the other pages are working properly so I won't include index.js

This is my code from app.js

const express = require('express');
const bodyParser = require('body-parser');
const cookieParser = require('cookie-parser');

const app = express();

app.use(bodyParser.urlencoded({extended: false}));
app.use(cookieParser());

const colors = [
    'red',
    'orange',
    'yellow',
    'green',
    'blue',
    'purple'
  ];

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

const mainRoutes = require('./routes'); //it goes automaticly to index.js
const cardRoutes = require('./routes/cards');

app.use(mainRoutes);
app.use('/cards', cardRoutes);

app.use((req, res, next) => {
    const err = new Error('Not Found');
    err.status = 404;
    next(err);
});

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

app.listen(3000, () => {
    console.log('The application is running on localhost:3000!');
});

This is the code from cards.js

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

router.get('/', (req, res) => {
    res.render('card', {
        prompt: "Who is buried in Grant's tomb?"
    });
}); 

module.exports = router;

When I try to go on the /cars route, I get this error:

RangeError: Invalid status code: 0

at ServerResponse.writeHead (_http_server.js:192:11)

at ServerResponse._implicitHeader (_http_server.js:157:8)

at ServerResponse.OutgoingMessage.end (_http_outgoing.js:573:10)

at ServerResponse.send (E:\PracticeGrounds\PracticeGrounds\Treehouse\Express Basics\flashcards\node_modules\express\lib\response.js:221:10)

at done (E:\PracticeGrounds\PracticeGrounds\Treehouse\Express Basics\flashcards\node_modules\express\lib\response.js:1004:10)

at Object.exports.renderFile (E:\PracticeGrounds\PracticeGrounds\Treehouse\Express Basics\flashcards\node_modules\pug\lib\index.js:422:12)

at View.exports.__express [as engine] (E:\PracticeGrounds\PracticeGrounds\Treehouse\Express Basics\flashcards\node_modules\pug\lib\index.js:465:11)

at View.render (E:\PracticeGrounds\PracticeGrounds\Treehouse\Express Basics\flashcards\node_modules\express\lib\view.js:135:8)

at tryRender (E:\PracticeGrounds\PracticeGrounds\Treehouse\Express Basics\flashcards\node_modules\express\lib\application.js:640:10)

at EventEmitter.render (E:\PracticeGrounds\PracticeGrounds\Treehouse\Express Basics\flashcards\node_modules\express\lib\application.js:592:3)