pya3rtを使ったLINE bot (python)で返答がこないエラーを解決したい

Question

前提・実現したいこと

pythonとherokuで、pya3rtを使ったLINEトークを作ろうとしていますが、下記エラーが出てLINEから返答が返ってきません。
調べてみたのですが、解決方法がわかりませんでした。
どなたかわかる方いましたらご教授をお願いします。

発生している問題・エラーメッセージ

2021-11-09T12:23:47.027990+00:00 app[web.1]: [2021-11-09 12:23:47,021] ERROR in app: Exception on /callback [POST]
2021-11-09T12:23:47.028016+00:00 app[web.1]: Traceback (most recent call last):
2021-11-09T12:23:47.028016+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
2021-11-09T12:23:47.028017+00:00 app[web.1]: response = self.full_dispatch_request()
2021-11-09T12:23:47.028017+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
2021-11-09T12:23:47.028017+00:00 app[web.1]: rv = self.handle_user_exception(e)
2021-11-09T12:23:47.028018+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
2021-11-09T12:23:47.028018+00:00 app[web.1]: reraise(exc_type, exc_value, tb)
2021-11-09T12:23:47.028018+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
2021-11-09T12:23:47.028019+00:00 app[web.1]: raise value
2021-11-09T12:23:47.028019+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
2021-11-09T12:23:47.028019+00:00 app[web.1]: rv = self.dispatch_request()
2021-11-09T12:23:47.028020+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
2021-11-09T12:23:47.028020+00:00 app[web.1]: return self.view_functions[rule.endpoint](**req.view_args)
2021-11-09T12:23:47.028021+00:00 app[web.1]: File "main.py", line 50, in callback
2021-11-09T12:23:47.028021+00:00 app[web.1]: handler.handle(body, signature)
2021-11-09T12:23:47.028021+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/linebot/webhook.py", line 259, in handle
2021-11-09T12:23:47.028022+00:00 app[web.1]: self.__invoke_func(func, event, payload)
2021-11-09T12:23:47.028022+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/linebot/webhook.py", line 271, in __invoke_func
2021-11-09T12:23:47.028022+00:00 app[web.1]: func(event)
2021-11-09T12:23:47.028023+00:00 app[web.1]: File "main.py", line 61, in handle_message
2021-11-09T12:23:47.028023+00:00 app[web.1]: reply_text = talkapi_response(push_text)
2021-11-09T12:23:47.028023+00:00 app[web.1]: File "main.py", line 70, in talkapi_response
2021-11-09T12:23:47.028023+00:00 app[web.1]: response = client.talk(text)
2021-11-09T12:23:47.028024+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/pya3rt/client.py", line 165, in talk
2021-11-09T12:23:47.028024+00:00 app[web.1]: return Talk.request(endpoint, apikey, query, callback)
2021-11-09T12:23:47.028024+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/pya3rt/talk.py", line 16, in request
2021-11-09T12:23:47.028025+00:00 app[web.1]: return response.json()
2021-11-09T12:23:47.028025+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/site-packages/requests/models.py", line 850, in json
2021-11-09T12:23:47.028026+00:00 app[web.1]: return complexjson.loads(self.text, **kwargs)
2021-11-09T12:23:47.028026+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/json/__init__.py", line 357, in loads
2021-11-09T12:23:47.028026+00:00 app[web.1]: return _default_decoder.decode(s)
2021-11-09T12:23:47.028026+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/json/decoder.py", line 337, in decode
2021-11-09T12:23:47.028027+00:00 app[web.1]: obj, end = self.raw_decode(s, idx=_w(s, 0).end())
2021-11-09T12:23:47.028027+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.8/json/decoder.py", line 355, in raw_decode
2021-11-09T12:23:47.028027+00:00 app[web.1]: raise JSONDecodeError("Expecting value", s, err.value) from None
2021-11-09T12:23:47.028027+00:00 app[web.1]: json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
2021-11-09T12:23:47.032800+00:00 app[web.1]: 10.1.32.177 - - [09/Nov/2021 12:23:47] "POST /callback HTTP/1.1" 500 -

該当のソースコード

main.py

# インポートするライブラリ
from flask import Flask, request, abort
from linebot import (
   LineBotApi, WebhookHandler
)
from linebot.exceptions import (
   InvalidSignatureError, LineBotApiError
)
from linebot.models import (
   FollowEvent, MessageEvent, TextMessage, TextSendMessage, ImageMessage, ImageSendMessage, TemplateSendMessage, ButtonsTemplate, PostbackTemplateAction, MessageTemplateAction, URITemplateAction
)
import os
import requests
import pprint
import pya3rt
import json

# ログを出力するため。
import logging
import sys

# 軽量なウェブアプリケーションフレームワーク:Flask
app = Flask(__name__)

#環境変数からLINE Access Tokenを設定
LINE_CHANNEL_ACCESS_TOKEN = os.environ["LINE_CHANNEL_ACCESS_TOKEN"]

#環境変数からLINE Channel Secretを設定
LINE_CHANNEL_SECRET = os.environ["LINE_CHANNEL_SECRET"]

line_bot_api = LineBotApi(LINE_CHANNEL_ACCESS_TOKEN)
handler = WebhookHandler(LINE_CHANNEL_SECRET)

# 必須ではないけれど、サーバに上がったとき確認するためにトップページを追加しておきます。
@app.route('/')
def top_page():
    return 'Here is root page.'

@app.route("/callback", methods=['POST'])
def callback():
   # get X-Line-Signature header value
   signature = request.headers['X-Line-Signature']

   # get request body as text
   body = request.get_data(as_text=True)
   app.logger.info("Request body: " + body)

   # handle webhook body
   try:
       handler.handle(body, signature)
   except InvalidSignatureError:
       print("Invalid signature. Please check your channel access token/channel secret.")
       abort(400)
   return 'OK'

# MessageEvent
@handler.add(MessageEvent, message=TextMessage)
def handle_message(event):
    #talk api
        push_text = event.message.text
        reply_text = talkapi_response(push_text)
        line_bot_api.reply_message(
            event.reply_token,
            TextSendMessage(text=reply_text)
        )

def talkapi_response(text):
    apikey = "<取得したapikeyです>"
    client = pya3rt.TalkClient(apikey)
    response = client.talk(text)
    return response['results'][0]['reply']

if __name__ == "__main__":
   port = int(os.getenv("PORT"))
   app.run(host="0.0.0.0", port=port)

requirements.txt

Flask==1.1.2
line-bot-sdk==2.0.1
requests==2.12.4
pya3rt==1.1

Procfile

web: python main.py

runtime.txt

python-3.8.5

試したこと

pipでline-bot-sdkなど全部インストールし、最新版にしています。

補足情報（FW/ツールのバージョンなど）

参考にしたサイト：
https://note.com/consultant1221/n/n7084879ae345
https://qiita.com/Threen/items/3a5162b3002d2c77c63b

環境：
MacOS Big Sur(Ver.11.6)