Sudden variable type shift?

I'm new to coding in roblox, and I've just put a playerdata save script in my game. The problem I'm having is where external scripts are supposed to update stats, the variables are being passed as booleans. I was doing math and everything was working, then it suddenly stopped and said I couldn't do math anymore.

the module script:

local PlayerStatManager = {}

local DataStoreService = game:GetService('DataStoreService')
local playerData = DataStoreService:GetDataStore('PlayerData')

local AUTOSAVE_INTERVAL = 60

local sessionData = {}

--the offending function
--sudden shift to boolean for no reason
function PlayerStatManager:ChangeStat(player, statName, changeValue)
	sessionData[player][statName] = sessionData[player][statName] + changeValue
	print(sessionData[player][statName])
end

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

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

local function setupPlayerData(player)
	local data = getPlayerData(player)
	if not data then
		sessionData[player] = {Money = 0, Experience = 0}
		savePlayerData(player)
		print('new player')
	else
		sessionData[player] = data
		print('existing user')
		print(sessionData[player])
	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

the function in my update script:

game.Players.PlayerAdded:connect(function(player)
	while wait(2) do
		PlayerStatManager:ChangeStat(player, 'Money', 5)
		PlayerStatManager:ChangeStat(player, 'Experience', 5)
	end
end)

the new error I'm getting:

12:27:02.727 - ServerStorage.playerData_Module:11: attempt to perform arithmetic on field '?' (a boolean value)

****EDIT Upon loading these exact scripts into a new place they work as expected, I get no errors and the saved values are changing. I have narrowed it down to this specific place as being broken.