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