Whaaaat?
Ignore that. The Lich doesn't even debuff defense.
I think the Zombies also accept Defense Debuffs. Their puke might debuff defense, but I'm not sure.
It's a mistake in the typing of allowed sets. The only thing the Lich debuffs is To-Hit. The Grave Knights debuff defense with their broadsword attacks, and debuff to-hit with their self-heal and ranged attacks. The Zombies only debuff To-Hit with their self-heal, but don't debuff anything else.
Always up for teaming with good players, so hit me up: @Deceivius and @Deceivius2
That's what I thought. Just wanted to make sure I wasn't missing something.
Thanks! 
It's an old bug with Petrifying gaze that carried over to the lich. At some point (CoH beta maybe) Pet gaze had a defense debuff component. They didn't remove it from the power they just turned off the effect (like how protector bots had dispersion bubble, and the commando had auto turret). The flag for defense debuff remained and it was only until recently that they fixed this (sometime between I15 and I16).
Level 50s: to many to remember at this point
I was reading through the Necro/ guides and one of them recommended putting Defense Debuff IO's in the Lich. Out of curiousity, I looked through the powers of the various pets, and I could not find a power on the Lich that gave a defense debuff... Am I missing something?
I know Defense Debuff enhancements can go into the Lich, but I'm not sure what they're enhancing. The Grave Knights have powers with defense debuff effects, but I couldn't find one for the Lich...
Cheers!