LampPost (OPEN/CLOSE) - attempt to index a nil value?

Error: Workspace.Map.LampPost.Script:53: attempt to index a nil value

Automatic open and close LamPost, WHY "attempt to index a nil value"??

local oh,om = 6,30	-- Open Time (hours,minutes)
local ch,cm = 18,00	-- Close Time (hours, minutes)

local l = game:service("Lighting")
if (om == nil) then om = 0 end
if (cm == nil) then cm = 0 end

function TimeChanged(lf,lr)
	local ot = (oh + (om/60)) * 60
	local ct = (ch + (cm/60)) * 60
	if (ot < ct) then
		if (l:GetMinutesAfterMidnight() >= ot) and (l:GetMinutesAfterMidnight() <= ct) then
			lf.Enabled = false
			lf.Parent.BrickColor = BrickColor.Black()
			lf.Parent.Material = Enum.Material.SmoothPlastic
			-------------------------------------------------
			lr.Enabled = false
			lr.Parent.BrickColor = BrickColor.Black()
			lr.Parent.Material = Enum.Material.SmoothPlastic
		else
			lf.Enabled = true
			lf.Parent.BrickColor = BrickColor.White()
			lf.Parent.Material = "Neon"
			-------------------------------------------------
			lr.Enabled = true
			lr.Parent.BrickColor = BrickColor.White()
			lr.Parent.Material = "Neon"
		end
	elseif (ot > ct) then
		if (l:GetMinutesAfterMidnight() >= ot) or (l:GetMinutesAfterMidnight() <= ct) then
			lf.Enabled = false
			lf.Parent.BrickColor = BrickColor.Black()
			lf.Parent.Material = Enum.Material.SmoothPlastic
			-------------------------------------------------
			lr.Enabled = false
			lr.Parent.BrickColor = BrickColor.Black()
			lr.Parent.Material = Enum.Material.SmoothPlastic
		else
			lf.Enabled = true
			lf.Parent.BrickColor = BrickColor.White()
			lf.Parent.Material = "Neon"
			-------------------------------------------------
			lr.Enabled = true
			lr.Parent.BrickColor = BrickColor.White()
			lr.Parent.Material = "Neon"
		end
	else
		print("TimeChange 'LampPostScript' error. ")
	end
end

for i,v in pairs(script.Parent:GetChildren()) do
	local lf = v:FindFirstChild("Left").Light.SurfaceLight
	local lr = v:FindFirstChild("Right").Light.SurfaceLight
	TimeChanged(lf,lr)
	game.Lighting.Changed:connect(function(property)
		if (property == "TimeOfDay") then
			TimeChanged(lf,lr)
		end
	end)
end