Nasus Wither bug

Last time while playing I noticed right after casting my wither, it simply didn't go off on enemy champions, rather the icon had its flashing comet around it (like if it were used, only it still had colour, and no cooldown timer), but actually clicking the icon would result in "you cannot cast this ability right now" for a couple of seconds. Now the wither generally applied, but a couple of times simply did what I have just described, and its effect didn't go off (which was frustrating to say the least). Note that I'm playing with smartcasts, so this isn't the case of me simply clicking W and forgetting to select the target- hell, the range indicator didn't even show up.

Anyone experienced something similar?