702.52. Dredge

Dredge is a static ability that functions only while the card with dredge is in a player’s graveyard. “Dredge N” means “As long as you have at least N cards in your library, if you would draw a card, you may instead mill N cards and return this card from your graveyard to your hand.”

A player with fewer cards in their library than the number required by a dredge ability can’t mill any of them this way.