As my group's usual Zendo Master, I have a lot of players fall into this trap. I like to train new players with one easy property like "A Koan Has The Buddah Nature If (and only if) it contains a red piece." Once they understand the rules, I jump to something like "A Koan Has The Buddah Nature Unless It contains exactly two pieces."
Switching from a positively-marked property (there is a simple feature which all these things have) to a negatively-marked property (there is a simple feature which all these things lack) can be pretty eye-opening.
I showed Zendo to a math professor once who fell smack into the 2-4-6 trap and tried to build as many white-marked koans as possible. He even asked why the game didn't punish people for just making the same koan over and over again, since it would be guaranteed to "follow the rule." I eventually managed to convey that the object of the game is to be able to tell me, in words, what you think the rule is. Since then I've been more explicit that "part of the game involves literally just saying, out loud, what you think defines the property." People always seem to think that the zendo is a sort of a silent lecture, when really it's more of a laboratory class.