问不一致Py 'NoneType‘对象没有属性'voice_channel’

EN

discord.ext.commands.errors.CommandInvokeError: Command raised an exception: AttributeError: 'NoneType' object has no attribute 'voice_channel'

from discord.ext import commands
import discord


class Voice(commands.Cog):
    def __init__(self, client):
        self.client = client

    @commands.command(name='join')
    async def join(self, ctx):
        channel = ctx.message.author.voice.voice_channel
        print(str(channel))



def setup(client):
    client.add_cog(Voice(client))