Method execution tests from AWS API Gateway console fail with internal error

The AWS API Gateway console allows you to navigate to your resource, choose "Test" and send a test request to your Lambda. When I tried this, the Lambda was throwing an error:

TypeError: Cannot read property 'if-modified-since' of null

Yet the API worked fine when accessed from a browser.

Turning up the logging, I saw this in the logs:

DEBUG	{
  message: 'SERVERLESS_EXPRESS:PROXY',
  event: '{\n' +
    "  resource: '/api/{resource+}',\n" +
    "  path: '/api/test',\n" +
    "  httpMethod: 'GET',\n" +
    '  headers: null,\n' +

Yes, it looks like headers is null.

There are two workarounds:

• In the method execution form, define any random header in the Headers field.
• Wrap the serverlessExpress function to provide a default value for headers:

exports.handler = function handler(event, context, callback) {
  return serverlessExpress({
    app
  })(
    {
      ...event,
      headers: event.headers || []
    },
    context,
    callback
  )
}

I know this is not a real-world issue, but I still wasted a fair amount of time trying to get to the bottom of it! I'm pretty sure I'm not doing anything odd in my code or deployment.

index.js

import express from 'express'

const app = express()
const router = express.Router()

router.get('/api/test', (req, res) => {
  res.json({ message: 'Hello World!' })
})
app.use('/', router)

export default app

CDK

    const backend = new Function(this, 'backend', {
      functionName: 'test',
      runtime: Runtime.NODEJS_14_X,
      environment: {
        AWS_NODEJS_CONNECTION_REUSE_ENABLED: '1'
      },
      handler: 'main.handler',
      code: Code.fromAsset(path.join(path.dirname(require.resolve('test')), 'build'))
    })

    api.root.addResource('api').addResource('{resource+}').addMethod('ANY', new LambdaIntegration(backend))

package.json

  "dependencies": {
    "@vendia/serverless-express": "^4.3.7",
    "express": "^4.17.1"
  }

Start at the serverlessExpress handler path and compare the AWS API Gateway console test event, where headers is null, with a normal browser request. Reproduce the failure using the provided index.js Express route and CDK API definition; done means the console test no longer raises the null-header error and the behavior is covered by a regression test.