Role adding method that fails with HttpException

gistfile1.txt

[Command ("cornfield")]
[Summary (                                   "Banish a user to the cornfield")]
public Task InvokeBanishmentAsync ([Summary ("The user to be banished.")] SocketUser miscreant,
                                   [Summary ("For how long are they to be banished.")]
                                   TimeSpan period)
{
     _ = Task.Run (async () =>
                   {
                      if (miscreant.IsBot)
                         return ;

                      await this.Context.DataService.SetCornfield (miscreant.Id, period) ;

                      IRole      roleTimeOut = this.Context.Guild.Roles.Single (r => r.Name == "Time Out") ;
                      IGuildUser guildUser   = await this.Context.Guild.GetUserAsync (miscreant.Id) ;
                      await guildUser.AddRoleAsync (roleTimeOut) ;

                      await this.ReplyAsync ($"To the cornfield with you, {miscreant.Username}!") ;
                   },
                   Program.TokenSource.Token) ;

    return Task.CompletedTask ;
}