MalformedResponse when asking for Permission - node.js

Question

I am struggling on creating a very basic example asking the user for permission. The setup is
Intents:
"request_permission":
- phrase "where am I?"
- action: "request_permission"
- events: <empty>
"user_info":
- phrase: <empty>
- action: "user_info"
- events: "actions_intent_PERMISSION"
Following the official example using the node.js fulfillment library, I write:
'use strict';
const functions = require('firebase-functions');
const {Permission, DialogflowApp} = require('actions-on-google');
const {WebhookClient} = require('dialogflow-fulfillment');
const {Card, Suggestion, SimpleResponse} = require('dialogflow-fulfillment');
const app = dialogflow({debug: true});
process.env.DEBUG = 'dialogflow:debug'; // enables lib debugging statements
exports.dialogflowFirebaseFulfillment = functions.https.onRequest((request, response) => {
const agent = new WebhookClient({ request, response });
const app = new DialogflowApp({request, response});
function welcome_intent(agent) {agent.add(`welcome intent`); }
function fallback_intent(agent) {agent.add(`fallback intent.`); }
function request_permission(agent){
let conv = agent.conv();
conv.ask(new Permission({
context: 'I need your location',
permissions: 'DEVICE_COARSE_LOCATION'
}))
agent.add(conv);
}
function user_info(agent){
if (app.isPermissionGranted()) {
const zipCode = app.getDeviceLocation().zipCode;
if (zipCode) {
app.tell(`your zip code ise ${zipCode}`);
} else {
app.tell('I cannot tell your zip code.');
}
} else {
app.tell('Without permission I cannot tell the zip code');
}
}
let intentMap = new Map();
intentMap.set('Default Welcome Intent', welcome_intent);
intentMap.set('Default Fallback Intent', fallback_intent);
intentMap.set('request_permission', request_permission);
intentMap.set('user_info', user_info);
agent.handleRequest(intentMap);
});
Still, when starting the simulator and activating my app and then asking "where am I?" I get
MalformedResponse
'final_response' must be set.
And yes, I did check the firebase console logs and the "Fulfillment"-Slider "Enable webhook call for this event" is on.
I wouldn't ask here if there would a V2 compatible example with a very basic permission request. I am aware of the answer in Dialogflow v2 API + Actions v2 API: MalformedResponse 'final_response' must be set