Discord.js - .addRole is not a function

Hello, I am creating a Discord Bot and I am trying to make an Mute Command, but I always get the same error.

Can you guys help me?

Some information:

Discord.js version: 12.0.0-dev
I am using Klasa
Klasa version: 0.5.0-dev

Code:

const { Command } = require('klasa');
const { MessageEmbed } = require('discord.js');

module.exports = class extends Command {

    constructor(...args) {
        super(...args, { description: 'Mute an user.' })
    }

    async run(msg, args) {
        if(!msg.member.hasPermission("MANAGE_MEMBERS")) return msg.channel.send("You can't use this command.");

        let MuteUser = msg.guild.member(msg.mentions.users.first() || msg.guild.members.get(args[0]))
        if(!MuteUser) return msg.channel.send("Can't find user!");

        let MuteReason = msg.content.split(" ").slice(2).join(" ");

        let MuteRole = msg.guild.roles.find(r => r.name === "Spammer");
        if(!MuteRole) return msg.channel.send("Can't find the Spammer role!");

        let MuteChannel = msg.guild.channels.find(guild => guild.name === 'bot-logs');
        if(!MuteChannel) return msg.channel.send("Can't find the #bot-logs channel.");

        if(MuteUser.roles.has(MuteRole)) return msg.channel.send("That user is already muted!.");

        MuteUser.addRole(MuteRole.id);

        return MuteChannel.send(new MessageEmbed()
            .setAuthor("Mute"|| 'Unknown', "http://wolfdevelopment.cf/BotSymbols/info.png")
            .setColor("#ff0000")
            .addField("Muted User", `${MuteUser}`)
            .addField("Muted By", `<@${msg.author.id}>`)
            .addField("Muted In", `${msg.channel}`)
            .addField("Time", `${msg.createdAt}`)
            .addField("Reason", `${MuteReason}`));
    }
}

discordjs / discord.js

Discord.js - .addRole is not a function #2980