ROBLOX: attempt to index nil with 'TweenSize'

I try to making roading bar but error "attempt to index nil with 'TweenSize'" how can I fix it?

local gui = script.Parent
local background = gui:WaitForChild("Background")
local bar = background:WaitForChild("Bar")
local filler = bar:WaitForChild("Filler",5)
local percentage = bar:WaitForChild("Percent")

for i = 1, 100 do
    wait(0.04)
    percentage.Text = i.."%"

    local formula = i/100

    filler:TweenSize(UDim2.new(formula, 0, 1, 0), Enum.EasingDirection.InOut, Enum.EasingStyle.Quart, 0.5, true)
end