Yeah. Blame Ephi for that. RTB is being handicapped.
This is because the minimum is 3, and 1 is smaller than that. It doesn't check if the input if valid after you've put it in. Instead, it corrects you as you go, which can be very frustrating.
On the next update, I'll change the RTB minimum to 1, but hardcode it to be 3. It will also have a /setBoundaryRadius command that will set the boundary's radius to the distance between you and the center.