Attempt to preform arithmetic (sub) on nil and number, Works in studio but not in game?

Ok so, im making a defense boost tool thing, in studio it works just fine but in game it breaks.

active = false
defenseMode = false
db = false
local char = game.Workspace:WaitForChild(script.Parent.Parent.Parent.Name)


script.Parent.Activated:Connect(function()
     previousHealth = char.Humanoid.Health
    if active == false and defenseMode == false then
        print("xd")
        active = true
        defenseMode = true
        wait(5)
        defenseMode = false
        wait(30)
        active = false
    end
end)        function onChanged()
        local Diffirence = tonumber(previousHealth) - char.Humanoid.Health
        if defenseMode == true then
            local GetPercent = 0
            local Part1 = Diffirence / 2
            print(Part1)
            local Part2 = Part1 / 2
            print(Part2)
            GetPercent = Part1 + Part2
            print(GetPercent)
            char.Humanoid.Health = char.Humanoid.Health + GetPercent
                GetPercent = 0
            end
        end
char.Humanoid:GetPropertyChangedSignal("Health"):Connect(function()
        if db == false then
            db = true
            onChanged()
            wait()
            db = true
        end
    end)