res.writeHead override not working

I'm trying to override the res.writeHead method in Node.js, though it's throwing an error. Here's the code:

const http = require('http');
http.createServer((req, res) => {
    const _writeHead = res.writeHead;
    res.writeHead = (...a) => {
        console.log('res.writeHead called!');
        _writeHead(...a);
    };
    res.writeHead(200, {
        'Content-Type': 'text/plain'
    });
    res.end('Hello, world!');
}).listen(2020);

res.writeHead called! is logged and then I get a TypeError: Cannot read property 'statusMessage' of undefined when a client connects. Why?

via Bennett