Karthus passive bug

Sometimes whenever Karthus's passive is triggered his "Lay Waste" ability will not be able to be used for about 1-4 seconds. This is makes it incredibly irritating for whenever someone is getting away with low health and you cant use your main ability. Iv'e had this bug happen to me more times than I could count, and i'm pretty sure a lot of Karthus mains have noticed this more than other people.