Master Assassin Question

[Greco]

I finally upgraded my rogue to level 50 in assassination. So now he is a master assassin, whcih means tha backstabing instantly kills every foe. I also tested the new feat against some crabs and spiders on level 13, and it works perfectly. Upon backstabing an "assasinated" text briefly appears (instead of hit points of damage performed) and the enemy is shred in pieces.
However, when I tried to assassinate a Warden on level 12, I found out that backstabing works as usual (a "backstabing" text appears along with the hit points of the damage performed). No matter how many backstabs I did, all worked as ordinary backstabs. So what I would like to ask the developers and other people who may know, is whether this is a bug or a feature. Are Wardens immune to the assasination feat, or it is just a bug? if it is a feature, are there other enemies which are immune to assassination? What about Goromorg, can they be assasinated?

[Disasterrific]

It's got to be a feature... I think they're the only creature, even slimes have backs!