I've Managed To Mess Up This Loop, How?

Hey guys. I've been trying to make a script that subtracts the position by .1 and the size by .2, so that it looks like sliding glass. This is my loop:

Shield = script.Parent

while script.Parent.Size.Y > 0.2 do
	Shield.Size.Y = Shield.Size.Y - .2
	Shield.CFrame.Y = Shield.CFrame.Y - .1
	wait(.05)
end
script.Parent.CanCollide = false

The error it throws is as follows: Y cannot be assigned to Script 'Workspace.Shield.Script', Line 4 Thanks in advance, Aurum