Thanks further checkingRoy_043 wrote: ↑Sat Sep 18, 2021 3:29 pm Maybe this needs to be checked. Changing the "Riser Thickness" from 0 to 1 fixes the OP's problem. And if "Riser Thickness" is zero Part_CheckGeometry does report problems. Your "Massive" solution just fixes the invalid solid issue in another way.
So, to summarize, "Structure" - "One Stringer" / "Two Stringer" are broken somehow, and no solid is resulted and thus the strange error. Making Riser / Tread Thickness > 0 make Stair return solid, thus also 'fix the problem'.