使用Express发出POST请求时出错

require('dotenv').config();

const express = require('express');
const cors = require('cors');
const utils = require('./utils');

const app = express();
const port = process.env.PORT || 4000;

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/db', {
    useNewUrlParser: true,
    useUnifiedTopology: true,
    useFindAndModify: false,
}).then(db => console.log('DB is connected! '))
    .catch(err => console.log(err));

const Users = require('./models/users');

app.use(cors());
app.use(express.json());

const fieldCheck = (field, res) => {
    return !field ? (res.status(400).json({
        error: true,
        message: `Invalid ${field}`
    })) : field
}

app.route('/scammer')
    .post(async (req, res) => {
        const userId = fieldCheck(req.body.user_id, res);

        await Users.updateOne({ 'user_id': userId }, { 'scammer': req.body.scammer });

        res.json({ 'error': false });
    });

app.listen(port, () => {
    console.log('Server started on: ' + port);
});