AWS Cognito PreventUserExistenceErrors setting with UserMigration Lambda in Golang - go

Question

I am trying to use PreventUserExistenceErrors on a Cognito pool with a user migration Lambda trigger. Cognito documentation says:
With user migration Lambda trigger, Cognito will return a simulated response for non existing users when an empty response was returned by the Lambda trigger.
https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-pool-managing-errors.html
I don't know how to get this to work. I isolated this by setting up a pool and attaching a simple trigger which always returns an empty response (we're writing the triggers in Go on the project):
package main
import (
"fmt"
"github.com/aws/aws-lambda-go/lambda"
)
func Handle(event interface{}) (interface{}, error) {
fmt.Println("nil")
// also tried:
// return struct{}{}, nil
// return "", nil
return nil, nil
}
func main() {
lambda.Start(func(event interface{}) (interface{}, error) {
return Handle(event)
})
}
I wrote a client to try to log in to the pool with a username that doesn't exist. With the PreventUserExistenceErrors enabled I expect the error to be the same as if the trigger was not attached to the pool:
Error executing "InitiateAuth" on "https://cognito-idp.eu-west-1.amazonaws.com"; AWS HTTP error: Client error: `POST https://cognito-idp.eu-west-1.amazonaws.com` resulted in a `400 Bad Request` response:
{"__type":"NotAuthorizedException","message":"Incorrect username or password."}
But I get a different error that shows the trigger failed:
HTTP error: Client error: `POST https://cognito-idp.eu-west-1.amazonaws.com` resulted in a `400 Bad Request` response:
{"__type":"UserNotFoundException","message":"Exception migrating user in app client 4i2oaatugssocd44d40kb55kni"}
I tried returning nil, empty string and empty struct and all three show the error from the trigger.
What's the correct way to return "empty response" from Cognito Lambda trigger in Go?