Attempted to concatenate a nil value? [SOLVED]

I'm making a ROBLOX implementation of CSS (to be used with GUIs) with a ModuleScript, and I'm getting an error. (These scripts are long but you shouldn't need to read the whole thing)

ModuleScript:

local module = {}

local function splitStylesheet(sheet)
	local split = {}
	
	local char = 1
	local line = 1
	for i = 1, string.len(sheet) do
		if string.sub(sheet, char, char) ~= "}" then
			table.insert(split, line, split[line]..string.sub(sheet, char, char))
			char = char + 1
		else
			line = line + 1
			char = char + 1
		end
	end
	
	return split
end

local function findSubstyles(dump)
	print("Splitting dump...")
	
	local open = 0
	local close = 0
	
	for brackets in string.gmatch(dump, "{") do
		open = open + 1
	end
	
	for brackets in string.gmatch(dump, "}") do
		close = close + 1
	end
	
	if open == close then
		local substyles = open
		
		return substyles
	elseif open > close then
		return "open"
	elseif close > open then
		return "close"
	else
		return "fatal"
	end
end

module.readStylesheet = function(name, dump)
	print("Reading stylesheet \""..name.."\"")
	
	if not name or not dump then
		error("Missing arg (function readStylesheet)")
		script:Destroy()
	end
	
	-- find number of substyles
	local substyles = findSubstyles(dump)
	
	if substyles == "open" then
		error("Assembler detected stray open bracket. Please check stylesheet")
		script:Destroy()
	elseif substyles == "close" then
		error("Assembler detected stray close bracket. Please check stylesheet")
		script:Destroy()
	elseif substyles == "fatal" then
		error("Assembler encountered fatal error. Please check stylesheet")
		script:Destroy()
	else
		print(substyles.." substyles found")
	end
	
	-- split substyles into a table
	local styletable = splitStylesheet(dump)
	
	print(styletable)
end

return module

Script I'm using the Module with (it's in a ScreenGui):

local css = require(game.Workspace.CSS)

css.readStylesheet("stylesheet1", "constructor1{background:none;}constructor2{background:none;}")

Output:

20:53:53.339 - Workspace.CSS:10: attempt to concatenate field '?' (a nil value)
20:53:53.359 - Stack Begin
20:53:53.359 - Script 'Workspace.CSS', Line 10 - upvalue splitStylesheet
20:53:53.360 - Script 'Workspace.CSS', Line 73 - field readStylesheet
20:53:53.360 - Script 'Players.Player.PlayerGui.PlayerGui.Main', Line 3
20:53:53.361 - Stack End

Anyone know what the problem is? Thanks to anyone who helps :)