Uncaught exception when adding member that is already part of the guild

[toms]

Which package is this bug report for?

discord.js

Issue description

I am attempting to add a member to my guild that is already an existing member like so

const addOpts = { accessToken, roles: roleIds, fetchWhenExisting: false };
const guildMember = await guild.members.add(discordId, addOpts);
if (!guildMember) {
    const existingGuildMember = await guild.members.fetch({ user: discordId });
    // more logic
}

but am getting the following uncaught exception:

TypeError: Cannot read properties of undefined (reading 'id')
    at GuildMemberManager._add (/node_modules/discord.js/src/managers/GuildMemberManager.js:39:52)
    at GuildMemberManager.add (/node_modules/discord.js/src/managers/GuildMemberManager.js:139:14)

The issue seems to be in this section here in the add function:

const data = await this.client.rest.put(Routes.guildMember(this.guild.id, userId), { body: resolvedOptions });
// Data is an empty Uint8Array if the member is already part of the guild.
return data instanceof Uint8Array
    ? options.fetchWhenExisting === false
        ? null
        : this.fetch(userId)
    : this._add(data);

The comment indicates that the returned result should be a Uint8Array if the member is already in the guild, but I am getting an empty ArrayBuffer instead.
I added the following log statements:

console.log('data', data);
console.log('data instanceof Uint8Array', data instanceof Uint8Array);
console.log('data instanceof ArrayBuffer', data instanceof ArrayBuffer);

and got the following output:

data ArrayBuffer { [Uint8Contents]: <>, byteLength: 0 }
data instanceof Uint8Array false
data instanceof ArrayBuffer true

And I do not think it's an oauth/bot permission issue because I'm able to successfully add a member to the guild if they are not already a member.

Code sample

No response

Versions

• discord.js - 14.12.1
• node - v18.18.1

Issue priority

Medium (should be fixed soon)

Which partials do you have configured?

No Partials

Which gateway intents are you subscribing to?

Guilds, GuildMembers

I have tested this issue on a development release

No response