Attempt to perform arithmetic on a nil value?

Hey guys, I've been using the wiki's example of player data manager which uses datastores for my game. Recently I made a few changes here and there so that other modules in my game can use it.

But since those changes, the script doesn't work, and the only change I've made is changed the Coins to 2 other things that my game requires.

The purpose of this script is basically store player data and update player data periodically to the respective database. I might rewrite it and totally modify it to my needs one day but right now something's not right. Everything's working on server so there shouldn't be that much delay in what loads and what doesnt but I'm not sure why there's a nil value there. Can someone help me out?

This is the script I have.

local AUTOSAVE_INTERVAL = 60
local DATASTORE_RETRIES = 3

local DataStoreService = game:GetService("DataStoreService")
local playerData = DataStoreService:GetDataStore("PlayerData")
local PlayerStatManager = {}
local sessionData = {}

function PlayerStatManager:ChangeStat(player, statName, changeValue)
	sessionData[player][statName] = sessionData[player][statName] + changeValue
	player[statName].Value = sessionData[player][statName]
end

local function dataStoreRetry(dataStoreFunction)
	local tries = 0
	local success = true
	local data = nil
	repeat
		tries = tries +1
		success = pcall(function()
			data = dataStoreFunction() 
		end)
		if not success then
			wait(1)
		end
	until tries == DATASTORE_RETRIES or success
	if not success then
		error('Not able to access the datastore! some of the data might not be stored!')
	end
	return success, data
end

local function getPlayerData(player)
	return dataStoreRetry(function()
		return playerData:GetAsync(player.UserId)
	end)
end

local function savePlayerData(player)
	return dataStoreRetry(function()
		return playerData:SetAsync(player.UserId, sessionData[player])	
	end)
end

local function setupPlayerData(player)
	local eggVal = Instance.new("NumberValue", player)
	local fireballVal = Instance.new("NumberValue", player)
	local success, data = getPlayerData(player)
	if not success then
		sessionData[player] = false
	else
		if not data then
			sessionData[player] = {Eggs = 0, Fireballs = 0}
			savePlayerData(player) 
			eggVal.Name = "Eggs"
			eggVal.Value = 0
			fireballVal.Name = "Fireballs"
			fireballVal.Value = 0
		else
			sessionData[player] = data
			eggVal.Name = "Eggs"
			eggVal.Value = sessionData[player]["Eggs"]
			fireballVal.Name = "Fireballs"
			fireballVal.Value = sessionData[player]["Fireballs"]
		end
	end
end


local function autosave()
	while wait(AUTOSAVE_INTERVAL) do
		for player, data in pairs(sessionData) do
			savePlayerData(player)
		end
	end
end




game.Players.PlayerAdded:Connect(setupPlayerData)
game.Players.PlayerRemoving:Connect(function(player)
	savePlayerData(player)
	sessionData[player] = nil
end)

spawn(autosave)
return PlayerStatManager

And, this is what I got for the error message

09:47:29.399 - ServerScriptService.Modules.PlayerStatManager:10: attempt to perform arithmetic on field '?' (a nil value)
09:47:29.400 - Stack Begin
09:47:29.400 - Script 'ServerScriptService.Modules.PlayerStatManager', Line 10 - method ChangeStat
09:47:29.401 - Script 'ServerScriptService.Controller.Main', Line 10
09:47:29.402 - Stack End

Why is this script not working? What can be possibly wrong with it? I don't see anything wrong personally.