This doesn't subtract health?

My code doesn't subtract the health. It's not the fact it's not the player's character it's hitting or something along the lines of that (I've already printed hit.Parent and it was the character). So I don't seem to see the problem

local b = script.Parent
local tool = b.Parent
local dmg = 10

b.Touched:connect(function(hit)
    local char = hit.Parent
    local hum = char:WaitForChild("Humanoid")
    if hum then
        local hth = hum.Health
        hth = hth - dmg
    end
end)

(Note it's an entirely new sword script mechanism for my game. I don't believe there is conflicting scripts. Unless hitting the bade is false if you are hitting during animation. I do not know.)