Dialogflow fulfillment webhook using Azure web app gives error

I have created fulfillment code for Dialogflow with Python Flask and deployed it as Azure Web App. Code is working fine with the url provided by Azure. But if I use the same url in Dialog Flow's Fulfillment webhook, I get an error saying "Webhook call failed. Error: UNKNOWN."

Here's my simple Python Flask Code which is deployed in Azure as Web App.

from flask import Flask, request, make_response, jsonify
app = Flask(__name__)

def hello():
    return "Hello World!"

def webhook():
    return jsonify("Webhook Successfull")

if __name__ == "__main__":

Webhook url in DialogFlow: enter image description here