Не отвечает бот на webhook

import flask
from flask import Flask, request
import telebot
from telebot import types

bot = telebot.TeleBot('***')
app = Flask(__name__)
bot.set_webhook('https://***.ngrok.io')


@app.route('/', methods=['POST'])
def webhook():
    if flask.request.headers.get('content-type') == 'application/json':
        json_string = flask.request.get_data().decode('utf-8')
        update = telebot.types.Update.de_json(json_string)
        bot.process_new_updates([update])
        return ''
    else:
        flask.abort(403)


@bot.message_handler(commands=['start'])
def start(message):
    buy = types.InlineKeyboardMarkup()
    buyButton = types.InlineKeyboardButton(text='ответ')
    buy.add(buyButton)
    bot.send_message(message.chat.id, 'Привет!', reply_markup=buyButton)


if __name__ == "__main__":
    app.run()