What happens when Trundle uses his ultimate on someone with negative armor?

If a champion has less than 0 armor, they receive bonus damage. The damage formula:

2-(100/(100-armor))

If Trundle uses his ult on a champion, he lowers their armor by a percent. This prevents him from dropping an enemy's armor below 0 with his ult alone. With a Black Cleaver or other form of armor reduction, it would be possible to do bonus damage (use the ult THEN get stacks on the cleaver!).

Edit: Armor penetration is applied in this order (source):

1. Percent reduction (Bushwhack / Pounce)
2. Flat reduction (The Black Cleaver, Puncturing Taunt, etc)
3. Flat penetration (The Brutalizer, Sunder Mastery, Runes of Desolation)
4. Percent penetration (Last Whisper)

I think that this question is more a curiosity than a real need. Trundle is an anti-tank and it would be quite silly using his ultimate on an unarmored squishy hero.

In addition percentage stolen is 15/20/25% and it is usually difficult to cause an enemy champion to have negative armor values lesser than -20. In this case have a 0 armor buff or a -3/-4/-5 armor debuff is quite irrelevant in terms of gameplay.