That is weird, since the card isn't supposed to work that way. It is supposed to take 50 HP, either from YOUR creatures (if you are the on playing it, of course), and if your creatures haven't enough HP, it takes the remaining from you. Now, that just doesn't make sense, since it's such a powerful creature at such a low cost...
Anyway, when that happened to me, there were indeed creatures in play, and, obviously, the closest avatar was my opponent's one, since he was the one playing the Dragon.