How to Fix 'attempt to perform arithmetic on nil' in Roblox

Quick answer: The error means one operand of a math expression was nil: a missing attribute, a failed tonumber, or a remote argument the client never sent. Initialize the value with a typeof check or an 'or 0' default before doing math on it.

Luau only does math on numbers. If either side of +, -, *, /, //, %, or ^ is nil, the VM stops and prints attempt to perform arithmetic on a nil value. Studio often names the operator too, such as (add) or (mul).

nil is not 0. It means there is no value. A missing attribute, a failed tonumber, an unassigned local, or a remote argument the client never sent all show up as nil. Adding to any of those is this error.

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What the error looks like in Studio

Here is a join-bonus script that crashes the first time a player arrives:

local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(player)
	local coins = player:GetAttribute("Coins")
	player:SetAttribute("Coins", coins + 50) -- coins is nil
end)

GetAttribute returns nil when the attribute does not exist yet. coins + 50 is then arithmetic on nil.

The same crash shows up when:

• You write kills = kills + 1 before kills has a number (and you skipped local).
• tonumber(textBox.Text) fails on "abc" and you add the result.
• An OnServerEvent handler does amount + bonus and the client omitted amount.
• You read data.Level from a table that has no Level key, then add XP to it.

If the Output says attempt to index nil with instead, you are past the number and into a missing Instance. Fix the Instance first. This page is only about math on nil.

Fix the value before you compute

Decide what "missing" should mean, then make that explicit. Starting coins are 0, not nil.

local Players = game:GetService("Players")

local STARTING_COINS = 0
local JOIN_BONUS = 50

Players.PlayerAdded:Connect(function(player)
	local coins = player:GetAttribute("Coins")
	if typeof(coins) ~= "number" then
		coins = STARTING_COINS
	end
	player:SetAttribute("Coins", coins + JOIN_BONUS)
end)

typeof is the Roblox-aware check. type(coins) == "number" also works for primitives. coins or 0 is a shorter default. For numbers it is usually fine, because 0 is truthy in Luau. Prefer the typeof branch when the value might be the wrong type, not only missing.

If the number lives on an Instance, do not assume the Instance exists:

local leaderstats = player:FindFirstChild("leaderstats")
local coinsValue = leaderstats and leaderstats:FindFirstChild("Coins")
if not coinsValue then
	return
end
coinsValue.Value += 1

FindFirstChild returns nil when the child is missing. Guard that nil, then do math on .Value. Indexing a missing child is a different error. Get the object, then compute.

How to find the nil

Print both sides of the operator:

print("coins", coins, typeof(coins))

The Output window tells you which argument is nil. Work backwards: which line was supposed to set it, and did that line run? On a remote handler, print every argument before you use it. If amount is nil, the client never sent a number (or you connected the wrong remote).

A NumberValue that exists still has a .Value of 0 by default, not nil. If .Value math is failing, you do not have the NumberValue. You have nil.

Once the types make sense, this error goes away. The free theory lesson on variables and data types covers nil, number, and typeof. First lessons are free. Premium unlocks the full curriculum. You can try a matching practice problem in the live editor, where plain-English errors point at the same mistake Studio is showing you.

Common places the nil hides

The error names the operator, not the variable, so the nil often comes from farther up than the failing line:

• A global that only gets its number inside one branch: kills = kills + 1 runs, but the branch that set kills = 0 never did.
• An optional remote argument: an OnServerEvent handler receives nil for arguments the client omitted, and amount + bonus then fails.
• A tonumber on player text: tonumber("abc") returns nil, and the next line does math on the result.
• A table read on the wrong key: data.Coins is nil when the save file uses coins, or the player has no save yet.

Next: Variables & Data Types

Understand loop structures in Luau, including numeric, while, generic pairs, and ipairs iterators.