Why is a "Cannot read property 'replace' of undefined" error being thrown on req.headers even with a null check on my NodeJS server?

I have a NodeJS server running on an elastic beanstalk instance. The logs of the elastic beanstalk instance is showing:

TypeError: Cannot read property 'replace' of undefined
    at Server.<anonymous> (/var/app/current/server.js:48:36)
    at Server.emit (events.js:311:20)
    at parserOnIncoming (_http_server.js:784:12)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:119:17)

The portion of code that is throwing this error is:

http.createServer(function (req, res) {
  if(req.headers.host){
    let httpsHost = req.headers.host.replace('8081', '8443');
    res.writeHead(301, {"Location": "https://" + httpsHost + req.url});
    res.end();
  }
}).listen(8081);

The purpose of this code is to force redirect all HTTP requests to HTTPS.

The server is working normally, but this error is getting thrown on a constant basis. I see no adverse effects on the server from this error, but it is making my logs look terrible.

1. Why is this error even happening? Why do some req.headers not have a host variable defined?
2. Why is my null check not preventing this error?