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 :)