Geier Reach Sanitarium has an ability to pay 2 colourless to have each player 'draw a card, THEN discard a card'. However, when Narset is in play the opponent may only draw one card each turn. This can be used to prevent the opponent from drawing cards if it is activated directly before draw step (which is overpowered imo) but also can be used to force the oppoenet to discard a card. However, if no card was drawn, the opponent is still forced to discard, despite the word "THEN" being present as a qualifier in the card effect. If no card is drawn, then no card should be discarded.