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.