Why does my int variable get declared as an instance variable?

local plrs = game.Players
local values = {}
local highestValue = 1

plrs.PlayerAdded:Connect(function(player)
    local inventoryFolder = Instance.new("Folder")
    inventoryFolder.Name = "BackpackInventory"
    inventoryFolder.Parent = player

    wait(1)
    while wait(1) do
        if inventoryFolder:FindFirstChild("Sand") then


            -- Create data for the blocks

            local sandData = Instance.new("Folder")
            sandData.Name = "SandData"
            sandData.Parent = inventoryFolder.Sand  

            local sandSellWorth = Instance.new("IntValue")
            sandSellWorth.Value = 1
            sandSellWorth.Parent = sandData
            sandSellWorth.Name = "SellWorth"

            local sandId = Instance.new("IntValue")
            sandId.Name = "SandId"
            sandId.Parent = sandData



            -- Give the sand an ID


            for i, v in pairs(inventoryFolder.Sand:GetChildren()) do

                table.insert(values,v.SandId.Value)

                for valueNumber, IDvalue in pairs(values) do
                    if IDvalue > highestValue then
                        highestValue = v
                    end
                end

            end

            sandId.Value = highestValue + 1

            highestValue = 0
            values = {}

        else

            local sandFolder = Instance.new("Folder")
            sandFolder.Parent = inventoryFolder
            sandFolder.Name = "Sand"

            local sandData = Instance.new("Folder")
            sandData.Name = "SandData"
            sandData.Parent = inventoryFolder.Sand  

            local sandSellWorth = Instance.new("IntValue")
            sandSellWorth.Value = 1
            sandSellWorth.Parent = sandData
            sandSellWorth.Name = "SellWorth"

            local sandId = Instance.new("IntValue")
            sandId.Name = "SandId"
            sandId.Parent = sandData
            sandId.Value = 1

        end
    end 
end)

And in the output it says:

ServerScriptService.Script:40: attempt to compare Instance < number

I've tested it out and it seems highestValue is considered an instance, for some reason. Using tonumber(highestValue) declares it nil.