m_Rail Balustrade is a macro object. It's used by the stair. Line 39 talks about baluster spacing. It's getting the idea that there are zero balusters, and when it tries to find the spacing, it divides by that zero.
There's also an independent railing object that uses that macro. 'Rail Balustrade' in 03 Concrete (
)/Concrete Structures/Stairmaker handrails. I tried to break that, but it seems to handle zero balusters OK, so the problem is probably in the stair's calling of the macro.
As you can see, this isn't a solution so much as a clue. I can't tell you exactly what to do, except to experiment with the baluster spacing.
Probably not much help,