How can i fix this error in this Day/Night cycle script?

So this is a day/night cycle script and the timeshift is set to 0.20 however after some time it gives this error ServerScriptService.Day/Night Cycle:44: attempt to perform arithmetic (sub) on number and nil, and it stays on night time for ever. However if the timeshift is set to 0.25 there is no problem and it runs forever any ideas how can this be solved?

--time changer
local mam --minutes after midnight
local timeShift = .20 --how many minutes you shift every "tick"
local waitTime = 1/30 --legnth of the tick
local pi = math.pi
--brightness
local amplitudeB = 1
local offsetB = 2
--outdoor ambieant
local var
local amplitudeO = 20
local offsetO = 100
--shadow softness
local amplitudeS = .2
local offsetS = .8

--color shift top
local pointer
				--	1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16  17  18  19  20  21  22  23  24
local rColorList = {000,000,000,000,000,255,255,255,255,255,255,255,255,255,255,255,255,255,255,000,000,000,000,000}
local gColorList = {165,165,165,165,165,255,215,230,255,255,255,255,255,255,255,245,230,215,255,165,165,165,165,165}
local bColorList = {255,255,255,255,255,255,110,135,255,255,255,255,255,255,255,215,135,110,255,255,255,255,255,255}

local r
local g
local b

while true do
	--time changer
	mam = game.Lighting:GetMinutesAfterMidnight() + timeShift
	game.Lighting:SetMinutesAfterMidnight(mam)
	mam = mam/60
	--brightness
	game.Lighting.Brightness = amplitudeB*math.cos(mam*(pi/12)+pi)+offsetB
	--outdoor ambient
	var=amplitudeO*math.cos(mam*(pi/12)+pi)+offsetO
	game.Lighting.OutdoorAmbient = Color3.fromRGB(var,var,var)
	--shadow softness
	game.Lighting.ShadowSoftness = amplitudeS*math.cos(mam*(pi/6))+offsetS
	--color shift top
	pointer= math.ceil(mam)
	r=((rColorList[pointer%24+1]-rColorList[pointer])*(mam-pointer+1))+rColorList[pointer]
	g=((gColorList[pointer%24+1]-gColorList[pointer])*(mam-pointer+1))+gColorList[pointer]
	b=((bColorList[pointer%24+1]-bColorList[pointer])*(mam-pointer+1))+bColorList[pointer]	
	game.Lighting.ColorShift_Top=Color3.fromRGB(r,g,b)
	--tick
	wait(waitTime)
end