Whispers of infinity Rallying cry Merc

Hi, i was messing around to find a way to get a reduced cost of the howlcrack on the merc, and i noticed that on him it doesn't cost anything, but witout that amulet he keep spamming cry and dies during a map 🥲 (wich is normal)

At least my merc has 4727 life, 383 ES, howlcrack should damage for like 709, and thanks to whispers it should cost instead energy shield wich it doesn't happen for some reason, maybe merc not actually using mana so they don't have an cost to skills