How to combine two tables? Where table A overwrites table B.

Table A = data (not Data) Table B = CompTable

I'm trying to combine A with B, where A overwrites B, but if something exists in B but not in A, B adds the value to A. I've tried several times and I'm currently lost..

"attempt to index field '?' (a nil value)"

local CompTable = {
	["Stats"] = {
		["Money"] = 100;
		["Exp"] = 0;
		["Placements"] = {
			["PropA"] = {
				["Rotation"] = 2;
				["Position"] = Vector2.new(1, 5)
			};
		};
		["Inventory"] = {
			["1"] = 10945;
		};
	};
	["Time"] = {
		["LastJoin"] = false;
		["LastCrate"] = false;
	};
	["Settings"] = {
		["DarkUITheme"] = false;
		["BlurFX"] = false;
		["ColorCorrectionFX"] = false;
	};
}

function module:combineTables(data)
	if not data or data == false then return CompTable end
	local DataTable = {}
	local pass, reason = pcall(function()
		for Index, DString in pairs(CompTable) do
			DataTable[tostring(Index)] = {}
			for Index2, DString2 in pairs(DString) do
				if data[tostring(Index)][tostring(Index2)] == true then --error
					DataTable[tostring(Index)][tostring(Index2)] = data[tostring(Index)][tostring(Index2)]
				else
					DataTable[tostring(Index)][tostring(Index2)] = DString
				end
			end
		end
	end)
	print(pass, reason)
	return DataTable
end