Bug with Smite, I smite does not always grant assists.

This actually happen to me a few times in summoners rift. I had Blue smite with cinderhulk. Champion almost dead I smite the champion a fraction of a second before my allies kill him, it hits, smite goes on cd but I do not get any assist for that kill. Is it a bug or is there a minimum dmg I need to do in order to get a assist? Seems more like a bug to me. Please let me know. Thank you.