Help with non working unit check

[Sauerkraut]

I don't really know how to explain this, but I am trying to fix an addon that allows either all units or a specified list of units. The all units part is working fine however when specific units are selected nothing is output to the game. There is no error it just does nothing.

From the addon

local function UnitisAllowed(unit)
	for key, val in pairs(addon.CONFIG.units) do
		if UnitIsUnit(unit, val) then return true end
	end
	return false
end

if eventType == "SPELL_AURA_APPLIED" and addon.BUFF_SPELLS[spellname] and isEnemy(destFlags) and (addon.CONFIG.allunits or UnitisAllowed(destName)) then

from the config

addon.CONFIG = {
	allunits = false, 
	units = {
		"target",
		"focus",
	},
}

Am I using the correct syntax for the check? I'm just not sure why it is failing.