WHY does this seemingly illogical error happen?

So I have this module, and whenever I try to call this function:

(around line 22)

-- Read data function
 function PlayerStatManager:GetData(player, statName)
    warn("DATA : ",unpack(sessionData))
    return sessionData[player.Name][statName]
end

it gives me this error:

ServerScriptService.SaveDataModule:22: attempt to concatenate a nil value

This is the entire module. It is a modified version of the one on the official Roblox wiki page

-- Setup table that we will return to scripts that require the ModuleScript.
PlayerStatManager = {}

-- Create variable for the DataStore.
DataStoreService = game:GetService('DataStoreService')
playerData = DataStoreService:GetDataStore('PlayerData2'.. game.PlaceId )

-- Create variable to configure how often the game autosaves the player data.
AUTOSAVE_INTERVAL = 60

-- Table to hold all of the player information for the current session.
sessionData = {}

-- Function the other scripts in our game can call to change a player's stats. This
-- function is stored in the returned table so external scripts can use it.
function PlayerStatManager:ChangeStat(player, statName, changeValue)
    sessionData[player.Name][statName] = sessionData[player.Name][statName] + changeValue
end

-- Read data function
 function PlayerStatManager:GetData(player, statName)
    warn("DATA : ",unpack(sessionData))
    return sessionData[player.Name][statName]
end

-- Function to retrieve player's data from the DataStore.
local function getPlayerData(player)
    return playerData:GetAsync(player.UserId)
end

-- Function to save player's data to the DataStore.
local function savePlayerData(player)
    playerData:SetAsync(player.UserId, sessionData[player.Name])
end

-- Function to add player to the sessionData table. First check if the player has
-- data in the DataStore. If so, we'll use that. If not, we'll add the player to
-- the DataStore.
local function setupPlayerData(player)
    local data = getPlayerData(player)
    if not data then
        -- DataStores are working, but no data for this player
        sessionData[player.Name] = {Money = 0,
                               MaxHealth = 100,
                               MaxShield = 100,
                               ["tools"] = {
                                 -- {"Sword","Classic sword",{}},
                                 {"Ax","Classic ax",{"Speedy","Strong"},{2,3}}
                               },
                               ["items"] = {
                                 {"Wood",5,"The stuff of trees"},
                                 {"Fish",3,"a limbless animal with gills and fins"}
                               }

                              }

        savePlayerData(player)
    else
        -- DataStores are working and we got data for this player
        sessionData[player.Name] = data
    end
end

-- Function to run in the background to periodically save player's data.
local function autosave()
    while wait(AUTOSAVE_INTERVAL) do
        for player, data in pairs(sessionData) do
            savePlayerData(player)
        end
    end
end

-- Bind setupPlayerData to PlayerAdded to call it when player joins.
game.Players.PlayerAdded:connect(setupPlayerData)

-- Call savePlayerData on PlayerRemoving to save player data when they leave.
-- Also delete the player from the sessionData, as the player isn't in-game anymore.
game.Players.PlayerRemoving:connect(function(player)
    savePlayerData(player)
    sessionData[player.Name] = nil
end)

-- Start running autosave function in the background.
spawn(autosave)

-- Return the PlayerStatManager table to external scripts can access it.
return PlayerStatManager


-- PlayerStatManager:ChangeStat(player, statName, changeValue)
-- PlayerStatManager:GetData(player, statName)
-- https://www.roblox.com/Thumbs/Asset.ashx?width=110&height=110&assetId=1818

what is annoying me is the fact that the script errors because it thinks sessionData is undefined when I try to call it in

warn("DATA : ",unpack(sessionData))

and sessionData is clearly defined a few lines above