Null koan

In Zendo, the null koan is the koan with zero pyramids. To minimize confusion, the null koan is used in the game only if the Master permits it. Some people believe that a koan with no pieces is not a koan at all.

Because the null koan looks so similar to a stray marking stone, the Master may choose some other object (such as a drink coaster) to represent the null koan.

Needless to say, there is only one null koan. That is, if there are two null koans in play, the Master is obliged to mark them identically, as they have no observable difference.

The null koan is the only one in which no piece is grounded.

According to the official rules, the null koan is not permitted:

Koans

Over the course of the game, players will create different arrangements of one or more pyramids on the table. Each arrangement is referred to as a "koan", pronounced "KO-ahn". Koans can be set up in any fashion, as long as they don't touch other objects or other koans.

(from http://www.koryheath.com/games/zendo )

What's wrong with it?

The null koan is never needed to make a game interesting. In fact, it more often leads to confusion and disagreement. For example, one potentially ambiguous rule is AKHTBN if every piece points at another piece. However this is more of a problem with the phrasing of such rules. The same problem would be displayed by a rule like AKHTBN if every red piece is grounded regardless of whether the null koan is allowed or not.