Question regarding geas

[ReynardTheFox]

I am DMing a game in fifth edition. Today a question came up about what happens when geas is cast under a false premise.
For example, a thief steals a player's gold, then gambles it all away. The player catchs the thief and demands his gold back. The thief lies and says he buried the gold under a tree. The player places a geas, commanding: "take me to the tree that the gold is buried under."
Since the thief doesn't know of any tree with gold buried under it, would the thief immediately take 5d10 damage; would the geas fail and the caster would know; or would the geas work, but with no way to fulfill the geas, there would there also be no way to "act in a manner directly counter to your instructions" so the thief could lead the player where ever he wished and take no damage?

[Jarlhen]

I don't know the RAW for it but I'd probably rule that the geas fails and the caster knows it failed.

[Gtdead]

You can either rule that the npc doesn't understand the command, or that he just sits there doing nothing since there is no such tree, but he still has the charmed condition till the spell ends ( so the player can have advantage when he asks where is the gold).

You could always alter the spell or make the player change the wording of the command to "bring me back the gold you stole". Then the npc could go and steal it back.