What happens if a replacement effect tries to replace something with an impossible action?

For example: Appa controls Notion Thief and Naga controls Spirit of the Labyrinth. After Appa has drawn for the turn, Naga cycles a card. Thief tries to replace the card draw, but Spirit means Appa can't draw and the replacement effect can't happen. Does the replacement fail, allowing Naga to draw a card? Or does the card draw get replaced by “nothing,” and no one draws?

The Spirit shuts down that player being able to draw once they're drawn a card that turn. And can't effect beat out can effects. Since Naga has already drawn for the turn, when they go to draw their next card, the Spirit prevents the draw, since the Spirit says they can't draw anymore cards that turn. Since the draw never happens, the Thief never gets a chance to apply and the net result is that neither player will draw a card.

101.2. When a rule or effect allows or directs something to happen, and another effect states that it can’t happen, the “can’t” effect takes precedence.
Example: If one effect reads “You may play an additional land this turn” and another reads “You can’t play land cards this turn,” the effect that precludes you from playing lands wins.

Nathan Long
Wizards.Com Boards NetRep