Hat that is a part with a decal giver trouble! Help?

This is a script that is supposed to give you a part on your head and weld it on to it

function GiveHat()

if HatOn == False then 

local I = Instance.new("Part")

local D = Instance.new("Decal")

local T = script.Parent.Parent.Parent.Parent:FindFirstChild("Head")

D.Parent = I

D.Texture = "http://www.roblox.com/asset/?id=149397688" 

I.Parent = T

I.CanCollide = false

I.Size = Vector3.new(0.2,1,0.2)

print("Part Created")

--Weld I and T

I.Position = T.Position + Vector3.new(0,1,0)

script.Parent.HatOn = true

else

local T = script.Parent.Parent.Parent.Parent:FindFirstChild("Head")

HatOn = false

T.I:Destroy()

end

end

script.Parent.MouseButton1Down:connect(GiveHat)

This is the exact code I'm having trouble with. Everytime I run it, it always says "attempt to index local "T"(a nil value)" and yes, script.Parent.Parent.Parent.Parent is the player.

also I don't know how to weld the 2 parts together