pretty clean object. surprised that it actually fails for no reason. temp.3dm (304.4 KB)
Actually, it’s not. It’s invalid (bad object). Use Check to see:
Rhino polysurface object is not valid.
brep.m_L loop is not valid.
end of brep.m_T[loop.m_ti=102]=(118.03,266.977) and start
of brep.m_T[loop.m_ti=103]=(30.7299,199.5) do not match.
brep.m_F face is not valid.
brep.m_L[face.m_li=21] is not valid.
ON_Brep.m_F is invalid.
If you extract the one bad surface - you can already see it’s bad by a trim line that goes into space - delete it and then Cap the object to close, the Split will work.