falling through a stair

[JD Zombie]

Using the right side round floating staircase from the castle set I've noticed that on the outside edge of the top stair if I brush up against the rail there I fall right through the stairs. It doesn't appear to happen with the left side version.

I've sort of worked around it by putting a small plane just under the surface of that step so when I fall through it catches me and as I keep moving up the step all is well again but has anyone else encountered this, is there a better fix?