Getting a wall when there's no square in the 1st floor?

[ZombiesLoveBrainiacs]

Each square can only have north & west walls - so if I want get the southern wall, i have to get the northern wall of the next square.

 

But on the first floor, getting the next square returns nil if that square is outside of the house.

What am I missing here? How does the game even place the wall in that spot when there's no square for it?