DialogFlow context is always empty in google assistant - node.js

Question

I've made a simple chatbot on dialogflow by referencing the dialogflow documentation which is based on Dialogflow's fulfillment library. The bot works good on integration with Google Assistant.
However when I started using context (getting and setting context) in the node.js code, the bot doesn't work on google assistant.
This line agent.context.get('iotcontext'); is null in google assistant but works good in dialogflow test console.
Following is my code:
'use strict';
var https = require ('https');
const functions = require('firebase-functions');
const {WebhookClient} = require('dialogflow-fulfillment');
const {Card, Suggestion} = require('dialogflow-fulfillment');
process.env.DEBUG = 'dialogflow:debug'; // enables lib debugging statements
exports.dialogflowFirebaseFulfillment = functions.https.onRequest((request, response) => {
const agent = new WebhookClient({ request, response });
console.log('Dialogflow Request headers: ' + JSON.stringify(request.headers));
console.log('Dialogflow Request body: ' + JSON.stringify(request.body));
function welcome(agent) {
agent.add(`Welcome to my agent!`);
}
function fallback(agent) {
agent.add(`I didn't understand`);
agent.add(`I'm sorry, can you try again?`);
}
function myIntentHandler(agent) {
console.log("Function myIntentHandler Run");
const codeContext=agent.context.get("iotcontext");
agent.add("Date spoken by user: "+codeContext.parameters["date.original"]);
}
// Run the proper function handler based on the matched Dialogflow intent name
let intentMap = new Map();
intentMap.set('Default Welcome Intent', welcome);
intentMap.set('Default Fallback Intent', fallback);
// intentMap.set('your intent name here', yourFunctionHandler);
intentMap.set('IoTIntent', myIntentHandler);
agent.handleRequest(intentMap);
});