add void/split space vertically issue

[junweitan]

I am trying to use the "add void/split space vertically" to create a return air plenum on me ceiling to for my room.
I go to the "ceiling" section and select "return air plenum", define a height,
Did not tick the split space vertically box, and selected "none" in the "floor section.

When I clicked ok, the model created has two return air plenum, on the ceiling and on the floor. From the basic modeling tutorial provided by iES, this does not seem to be the intended outcome.