Sylas: Kingslayer (W) can be used while below the threshold, regardless of mana

I was playing Sylas today, and I ran out of mana and was low on health. All of my abilities had the normal blue water-droplet appearance - except Kingslayer (W). I was below the health threshold, so I was getting the new glowing icon to indicate that, and I thought "Surely, this is just a visual bug, I can't actually use it, the icon is just wrong."

But no. I could use it at no mana cost as long as I was at low health. It cost mana while I had enough health, but below that threshold, it cost nothing.

Honestly, this would be an interesting buff for Sylas, though unintentional. Having his ability that is designed to save him at low health have no mana cost when he's desperate for health sounds great.