为什么我的 discord 机器人在我的服务器中没有响应

Question

当我输入我的 discord 服务器时，机器人没有响应，但是当我dm 机器人时它会响应。 每当发送 discord 消息时，我的代码都会出错。

 @client.event
    async def on_message(message):
        # Make sure bot doesn't get stuck in an infinite loop
        if message.author == client.user:
            return

        # Get data about the user
        username = str(message.author)
        user_message = str(message.content)
        channel = str(message.channel)

        # Debug printing
        print(f"{username} said: '{user_message}' ({channel})")

        # If the user message contains a '?' in front of the text, it becomes a private message
        if user_message[1] == '?':
            user_message = user_message[1:]  # [1:] Removes the '?'
            await send_message(message, user_message, is_private=True)
        else:
            await send_message(message, user_message, is_private=False)

Ignoring exception in on_message`
Traceback (most recent call last):
  File "/home/runner/Discord-Bot/venv/lib/python3.8/site-packages/discord/client.py", line 343, in _run_event
    await coro(*args, **kwargs)
  File "/home/runner/Discord-Bot/bot.py", line 38, in on_message
    if user_message[0] == '?':
IndexError: string index out of range