[GAMEPLAY] Amumu's tears (w) dont turn off and still procs damage on 0 mana.

Something I noticed on Amumu while playing him in the jungle, while on 0 mana the W doesn't automatically turn off and still does damage to any enemies around Amumu. Other skills are obviously unable to be used as he is on 0 mana. Since Amumus W is a spell that costs mana/second it shouldn't still be running after he is out of mana. Much like how Karthus E turns off when he's oom. This isn't an issue to gameplay at all but I do think it might be a bug Riot might want to address.