Simple Gambit problem

Meenu

I want a cube with a circular inlet on its top face!

I tried it in Gambit by making a cube(volume) and then subtract a circle from top face but it doesn't allow and says "cannot subtract a face from upper topology"

Also, i tried by starting from vertices and made 6 faces of cube then subtracting circle from top face but then when i ask it to convert it into volume it gives an error "Is this an open shell"

my guess is that it considers it a solid volume and making a circular inlet in top face does not make sense but then how do tell gambit that i have air in this volume??

.....What is the way to get around with this??