ALB responds with HTTP code 502

Hey, I am attempting to deploy a small Express app to AWS as a Lambda connected to ALB via target group.

const serverless = require('@codegenie/serverless-express')
const express = require('express')

const app = express()

app.get('/200', function(req, res) {
    res.json({ message: 'all good' })
})

app.get('/404', function(req, res) {
    res.status(404).json({ message: 'not found' })
})

app.post('/test', function(req, res) {
    res.sendStatus(400)
})

module.exports.handler = serverless({ app })

I am able to successfully make a request to /200 and see the JSON response but I am not successful with requests that modify the status. In those I just see this response from ALB:

<html>

<head>
    <title>502 Bad Gateway</title>
</head>

<body>
    <center>
        <h1>502 Bad Gateway</h1>
    </center>
</body>

</html>

Can I please ask for guidance how would I go about debugging this? I don't see anything useful in my Cloudwatch logs of the Lambda function. Should I enable ALB logs?

Thank you.

CodeGenieApp / serverless-express

ALB responds with HTTP code 502 #688