Passing world border kicks player on single-player

If a player passes the newly-implemented world border, they will be kicked from their own game due to an illegal position. Tested on gamemodes 0 and 1.

What I expected to happen was...:
I expected to not be able to cross the border in either gamemode. If i did cross the border somehow (by using the /tp command?) i expected to either be killed or teleported back inside the world.

What actually happened was...:
I was kicked from a single-player game for having an illegal position. I was then brought to the multiplayer server selection screen.

Steps to Reproduce:
1. Go to the world border.
2. Cross the world border.