Moving people to voice channels (Discord.net) : I seam to have this weird error and I don't know how to fix it.

await userName.ModifyAsync(x =>
            {

                x.Channel = Program.client.GetChannel(588025239103995904) as IVoiceChannel;

            });

Error: Cannot implicitly convert type 'Discord.IVoiceChannel' to 'Discord.Optional<Discord.IVoiceChannel>'