attempt to iterate over a nil value

What the message means

ServerScriptService.Inventory:21: attempt to iterate over a nil value

Other forms of the same message:

• attempt to iterate over a number value
• attempt to iterate over a string value

Luau lets you loop over a table directly (for k, v in inventory do). When the value after in is not a table, it has nothing to walk through and stops. With pairs(x) or ipairs(x) the same mistake gives invalid argument #1 to 'pairs' (table expected, got nil) instead.

The type in the message is what Luau found: nil most of the time, number or string when you looped over a single value by mistake.

Causes and fixes

Three things cause this error in practice. Each one below has the smallest code that triggers it, then the fix.

• Player data has not loaded yet
• A function did not return the table
• An attribute was looped over

Player data has not loaded yet

If a table only exists after a data store load, code that runs earlier (a shop opening, a RemoteEvent) loops over nil.

local inventories = {}

local function listItems(userId)
	-- before the load sets inventories[userId]: attempt to iterate over a nil value
	for itemName, count in inventories[userId] do
		print(itemName, count)
	end
end

listItems(12345)

Check the table first and return early, or fall back to an empty table with or {} when "nothing yet" is a fine answer.

local inventories: { [number]: { [string]: number } } = {}

local function listItems(userId: number)
	local inventory = inventories[userId]
	if not inventory then
		warn(`[Inventory] not loaded for {userId}`)
		return
	end
	for itemName, count in inventory do
		print(itemName, count)
	end
end

listItems(12345)

A function did not return the table

A helper that builds a list but forgets return gives back nil. So does one that returns early on some path.

local function getSpawns()
	local spawns = {}
	for _, child in workspace:GetChildren() do
		if child:IsA("SpawnLocation") then
			table.insert(spawns, child)
		end
	end
	-- forgot: return spawns
end

for _, spawnPart in getSpawns() do
	print(spawnPart.Name)
end

Return the table on every path. A return type in the function signature makes the Script Editor warn you when one path returns nothing.

local function getSpawns(): { SpawnLocation }
	local spawns: { SpawnLocation } = {}
	for _, child in workspace:GetChildren() do
		if child:IsA("SpawnLocation") then
			table.insert(spawns, child)
		end
	end
	return spawns
end

for _, spawnPart in getSpawns() do
	print(spawnPart.Name)
end

An attribute was looped over

Attributes hold single values (text, numbers, booleans and a few Roblox types), never tables. GetAttribute returns nil when the attribute is not set, and a list saved as text is a string, so looping over it fails with nil or string in the message.

local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(player)
	-- not set yet: attempt to iterate over a nil value
	for _, badge in player:GetAttribute("Badges") do
		print(badge)
	end
end)

Keep lists in a table on the server (or in a folder of values). If you must store a list in an attribute, save it as JSON text and decode it, checking the type at each step.

local HttpService = game:GetService("HttpService")
local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(player: Player)
	local raw = player:GetAttribute("Badges")
	if typeof(raw) ~= "string" then
		return -- no badges saved yet
	end
	local ok, list = pcall(HttpService.JSONDecode, HttpService, raw)
	if ok and typeof(list) == "table" then
		for _, badge in list do
			print(badge)
		end
	end
end)

How to find the line that causes it

1. Open Output (Window menu, or the Script tab), clear it with Ctrl+K (⌘K on Mac), and press Play.
2. Find the first red line. It starts with the script's place in the Explorer and the line number, such as ServerScriptService.Shop:12:.
3. Click the red line. Studio opens that script with the cursor on the failing line.
4. Read the lines between Stack Begin and Stack End under it: they list the calls that led there, newest first, so you can see which function called the broken one.
5. Still not sure which value is wrong? Add print(typeof(x), x) just above the line, or set a breakpoint on it and read the values in the Watch window.

Questions

Sources and further reading

• Roblox Creator Docs: tables
• Roblox Creator Docs: Instance:GetChildren
• Luau: generalized iteration

How the code was checked: every block marked Fixed passes Luau's strict type checker against Roblox's API definitions (luau-lsp, 23 October 2026), and every block marked Breaks compiles and fails only when it runs. The message wording was checked by running the broken code with the Luau command line tool and against the message strings in Roblox Studio itself on 23 October 2026. A type check catches misspelt APIs and wrong types, not game logic, so playtest in Studio before you publish.