Lissandra's Iceborn Subjugation Bug

Hey guys,

I just played a game as Lissandra against a Karthus, and I don't believe that there was an ice thrall after I killed Karthus, probably because it might conflict with his passive? I'm not sure if this was intended or if this also occurs with other champions that have a postmortem passive as well (ie. Aatrox or Sion).

Thanks.