Discord Bot Node JS简单错误.send

在学习本教程时，我遇到了一个奇怪的问题 . 构建一个JS Discord bot，实际上只有33行，并且它的抛出错误是关于.send未定义的 . 我已经四处搜索了，我找不到任何有助于更接近解决问题的方法 .

const fs = require("fs");
const Discord = require("discord.js");
const client = new Discord.Client();
const config = require("./config.json");

client.login(config.token);

client.on("ready", () => {
  client.user.setGame(`on ${client.guilds.size} servers`);
  console.log(`Ready to serve on ${client.guilds.size} servers, for ${client.users.size} users.`);
});

client.on("guildMemberAdd", (member) => {
    console.log(`New User ${member.user.username} has joined ${member.guild.name}` );
    member.guild.defaultChannel.send(`${member.user} has joined this server`);
});

client.on("message", (message) => {

    if (!message.content.startsWith(config.prefix) || message.author.bot) return;

    if (message.content.startsWith(config.prefix + "ping")) {
        message.channel.send("pong!");
    } else

    if (message.content.startsWith(config.prefix + "foo")) {
        message.channel.send("bar!");
    }
});

  client.on("error", (e) => console.error(e));
  client.on("warn", (e) => console.warn(e));
  client.on("debug", (e) => console.info(e));

运行时，console.log可以毫不费力地工作，但是默认通道的消息会在PowerShell中引发以下错误

C:\Users\super\Desktop\autoslap\mybot.js:18
        member.guild.defaultChannel.send(`${member.user} has joined this server`);
                                   ^

TypeError: Cannot read property 'send' of undefined

任何帮助将不胜感激，对可能是如此简单的东西感到沮丧 .