A.            Section R301 - Design criteria.  See this section of the IRC except as provided below:...

            F.            Section R311.7.5.1 Risers.  The riser height shall be not more than eight inches (203mm).  ... Open risers are permitted provided that the openings located more than 30 inches (762 mm), as measured vertically, to the floor or grade below do not permit the passage of a four-inch-diameter (102 mm) sphere. Exceptions: (1) The opening between adjacent treads is not limited on spiral stairways. (2) The riser height of spiral stairways shall be in accordance with Section R311.7.10.1.
                G.            Section R311.7.5.2 Treads.  The tread depth shall be not less than nine inches (229mm)....


Interesting.  Note, the 4" sphere rule seems to be applied to open risers in NMAC.
However, also interesting, rise can be 8", tread 9", not the 7.75 and 10" limits of IRC

So...  NM could have issue with my open risers.