Random terrain gen problems?

So ive made this script where it duplicates 6x6 stud parts onto a 300x36 stud area. Every time a new part is duplicated, Var increases by 1. When Var reaches 300, the area stops generating. Then a function is activated so that each duplicant has its size increases randomly (in the y direction). The problem is, the output reads: bad argument #1 to 'pairs' (table expected, got nil). If you don't understand, please test the script for yourself:

Pos = game.Workspace.Model.Part.Position
x = 0
y = 0
z = 0
Var = 0
Var1 = 0
function Random()
	for i,v in pairs(Parts) do
	v.Size = Vector3.new(6,math.random(1,5),6)
	end
end
repeat
	wait(0.01)
	x = x + 6
	Var = Var + 1
	Var1 = Var1 + 1
	local NewPart = game.Workspace.Model.Part:Clone()
	NewPart.Anchored = true
	NewPart.Parent = game.Workspace.Model
	NewPart.Position = Pos + Vector3.new(x,y,z)
	if Var1==50 then
		Var1 = 0
		x = 0
		z = z + 6
	end
until Var==300 do
Random()
Parts = game.Workspace.Model:GetChildren()
end