Why is this error displaying 'Illegal arguments: undefined, string'?

The problem has been solved. Postman was sending the request as in 'text' format instead of 'JSON' format and as such the backend couldn't make sense of data. Every worked fine when changed the settings on my Postman from 'text' to 'JSON'.

controller

const express = require('express');
const router = express.Router();
const bcrypt = require('bcryptjs');
const jwt = require('jsonwebtoken');
const config = require('config');
const auth = require('../middleware/auth')
const { check, validationResult } = require('express-validator');
const User = require('../models/User');
const UserModel = require('../models/User');


// @route   POST api/users, Register a users; access   Public

router.post('/', [
    check('userName', 'Please add user name').not().isEmpty(),
    check('email', 'Please include a valid email').isEmail(),
    check('password', 'Please enter a password with six or more characters').isLength({ min: 5 })
    ],
  async (req, res) => {
    const errors = validationResult(req);
    if(!errors.isEmpty()) {
        return res.status(400).json({ errors: errors.array() });
    }
    
    const  { userName, firstName, lastName, email, password, dateOfBirth, dateOfRegistration, userCategory } = req.body;

    try {
        let user = await User.findOne( { email });

        if (user) {
            return res.status(400).json({ msg: 'User already exist'})
        }
        
        user = new User({
            userName,
            firstName,
            lastName,
            email,
            password,
            dateOfBirth,
            dateOfRegistration,
            userCategory
        });

        const salt = await bcrypt.genSalt(10);

        user.password = await bcrypt.hash(password, salt);

        await user.save();

        
        const payload = {
            user: {
                id: user.id
            } 
        }

      
    } catch (err) {
        console.error(err.message);
        res.status(500).send('Server Error')  
    }
   }
  );

module.exports = router;

server.js

const express = require('express');
const mongoose = require('mongoose');
const path = require('path');

// api require routes
const users = require('./routes/users');
// const auth = require('./routes/auth');
// const students = require('./routes/studentsSub');
// const workers = require('./routes/workersSub');


const app = express();

// database connection
const connectDB = async () => {
    try {
        await mongoose.connect('mongodb+srv://nawill:[email protected]/myFirstDatabase?retryWrites=true&w=majority', 
        {
            useNewUrlParser: true,
            useCreateIndex: true,
            useUnifiedTopology: true,
            useFindAndModify: false  
        });
        console.log('MongoDB Connected ...')
    } catch (err) {
        console.error(err.message);
        process.exit(1);
    }
};
connectDB();

//Middleware
app.use(express.json({ extended: false }));
// app.get('/', (req, res) => res.json ({ msg: 'Hello node project'}));


//Routes
app.use('/api/users', users );
// app.use('/api/auth', auth );
// app.use('/api/students', students );
// app.use('/api/workers', workers );


//Start Server
app.listen(3000, ()=> console.log("Server started on 3000"))