Why will "CustomizedTeleportUI = true" not actually work? (short)

In my main place, I have a lobby which teleports the player to the actual game. I've made a small GUI display to take the place of the (rather ugly) default ROBLOX teleport sequence, and have placed this at the top of my script:

game:GetService('TeleportService').CustomizedTeleportUI = true

However, it will not function. I've tried changing it to false, keeping it at true, placing just CustomizedTeleportUI = true on the next line -- nothing works!

Thank you for reading, and I hope to find why this isn't working.