Historical Rules · 1 May 2011
Rule 609.3
Official source text from the 1 May 2011 Rules release
If an effect attempts to do something impossible, it does only as much as possible. Example: If a player is holding only one card, an effect that reads "Discard two cards" causes him or her to discard only that card. If an effect moves cards out of the library (as opposed to drawing), it moves as many as possible.