"Attempt to call a nil value"?

Hierarchy

Model
>Script
>Open
>>U1
>>U2
>>U3
>>ClickDetector
>Closed
>>U1
>>U2
>>U3
>>ClickDetector

local openy = script.Parent.Open
local closed = script.Parent.Closed
openy.ClickDetector.MouseClick:connect(close)
closed.ClickDetector.MouseClick:connect(open)

function close()
    openy.U1.Transparency = 1
    openy.U2.Transparency = 1
    openy.U3.Transparency = 1
    openy.U1.CanCollide = false
    openy.U2.CanCollide = false
    openy.U3.CanCollide = false
    closed.U1.Transparency = 0
    closed.U2.Transparency = 0
    closed.U3.Transparency = 0
    closed.U1.CanCollide = true
    closed.U2.CanCollide = true
    closed.U3.CanCollide = true
end

function open()
    openy.U1.Transparency = 0
    openy.U2.Transparency = 0
    openy.U3.Transparency = 0
    openy.U1.CanCollide = true
    openy.U2.CanCollide = true
    openy.U3.CanCollide = true
    closed.U1.Transparency = 1
    closed.U2.Transparency = 1
    closed.U3.Transparency = 1
    closed.U1.CanCollide = false
    closed.U2.CanCollide = false
    closed.U3.CanCollide = false
end

Not sure why it calls a nil value.