Sitting on your yoga/pilates ball thingy at work when the boss man walks in your office.

I was unsure of the protocol. Should I have gotten up and into my regular chair? I considered trying to turn to face him while sitting on the ball but feared that I would either a) fall off or b) make some kind of fart-like squeaky noise, which would have then rendered the interaction even more horribly awkward than it already was.