Grinding in Arathi Highlands, a Witherbark Headhunter cast "Wither Touch" on me, and I got the error:
Interface\Addons\ag_UnitFrames\ag_UnitClass.lua:1356: attempt to compare number with nil.
I had one other debuff on me (Fevered Fatigue), and two buffs (MOTW and Thorns).
Just had this happen again (exactly the same error, when a headhunter cast "Wither Touch" on me).
Just had this cast on me with no other debuffs, and I didn't get the error.