Kalista's w passive not granting assists

It would seem that procing Kalista's w passive as the support does not grant an assist. For example, if Kalista is dueling a champ and barely loses, then the support swoops in and auto-attacks for the kill (and procing w passive) will net a kill for Kaista but nothing for the support. I died to a Teemo and Sona, my soul-buddy, auto-attacks to finish him but is granted no assist. I do not know if it is intentional or a bug (I suspect the latter), but I figured I would bring it to light.