attempt to perform arithmetic (add) on nil and number

What the message means

ServerScriptService.Rewards:11: attempt to perform arithmetic (add) on nil and number

Other forms of the same message:

• attempt to perform arithmetic (add) on Instance and number
• attempt to perform arithmetic (sub) on string and number
• attempt to perform arithmetic (mul) on table and number

Read the two types in the message in order: they are the left and right side of the operator. In coins + 1, (add) on nil and number means coins was nil and 1 was the number.

Luau never treats a missing value as 0, and it only turns text into a number when the text looks like one. Do both on purpose: (value or 0) for a missing number, tonumber(text) for text.

Causes and fixes

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

• A table value was never set (nil)
• An IntValue was used without .Value
• The value is text, from a TextBox or an attribute

A table value was never set (nil)

Reading a key that is not in a table gives nil. Adding to it, as in kills[player] += 1 the first time a player scores, fails because there is nothing to add to yet.

local kills = {}

local function addKill(player)
	-- first kill: kills[player] is nil
	-- attempt to perform arithmetic (add) on nil and number
	kills[player] += 1
end

addKill("Builderman")

Start the value at 0 with (kills[player] or 0), or set it when the player joins.

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

local function addKill(playerName: string)
	kills[playerName] = (kills[playerName] or 0) + 1
end

addKill("Builderman")

An IntValue was used without .Value

leaderstats.Coins is an IntValue object, not a number. Doing maths on the object gives (add) on Instance and number.

local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(player)
	local coins = player:WaitForChild("leaderstats"):WaitForChild("Coins")
	-- attempt to perform arithmetic (add) on Instance and number
	coins = coins + 10
end)

Read and write .Value on the object.

local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(player: Player)
	local leaderstats = player:WaitForChild("leaderstats", 10)
	local coins = leaderstats and leaderstats:WaitForChild("Coins", 10)
	if coins and coins:IsA("IntValue") then
		coins.Value += 10
	end
end)

The value is text, from a TextBox or an attribute

A TextBox's Text is always a string, even when the player typed digits. A string with non-digit text in maths fails with (mul) on string and number. Luau quietly converts text that looks like a number, such as "5", which is why the bug only shows up when a player types something else. Convert it on purpose instead.

local box = script.Parent

box.FocusLost:Connect(function()
	-- "abc" or "" in the box: attempt to perform arithmetic (mul) on string and number
	local doubled = box.Text * 2
	print(doubled)
end)

tonumber turns text into a number, or nil when it is not one. Check the result before you use it.

local box = script.Parent :: TextBox

box.FocusLost:Connect(function()
	local amount = tonumber(box.Text)
	if amount == nil then
		box.Text = "Type a number"
		return
	end
	print(amount * 2)
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: operators
• Roblox Creator Docs: tonumber
• Roblox Creator Docs: TextBox

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.