Teaching Yoga to Pessimists

What should a Yoga teacher do when confronted with a pure pessimist? Should a Yoga instructor take on this "worst case scenario?" Teaching Yoga to negative people is a rarity, but it happens. A pessimist can contaminate an entire room of optimists - if the negative energy is strong enough. Needless to say, a pessimist should spend some time with inspirational audio books and practice Yoga.