Zilean's tool tip on his W says it costs no mana, when it still does [bug] [tooltip]

Zilean's tool tip on his W says it costs no mana, when it still does. Not sure if it is meant to cost no mana or if the tool tip is just inaccurate. It has done this every game I have played Zilean since the preseason https://i.gyazo.com/d606a59485172f2fe52a9749c5a376a1.png https://i.gyazo.com/50d9e99526c8d15ed6cdbfb0b3f7d2ef.png