Why won't this work when it did on another script? "attempt to perform arithmetic"

I am not the best at scripting so I took lines 16-18 from a different script which happens to be the problem here. Someone has explained to me about values and objects but never clearly enough to be understood!

The output on studio showed this: ": attempt to perform arithmetic on local 'decal' (a nil value)" Here is the script;

-- Sodaghost13

local frame = script.Parent
local tl = frame:WaitForChild("TextLabel")
local input = frame:WaitForChild("Input")
local settings = frame:WaitForChild("Settings")
local confirm = frame:WaitForChild("Confirm")
local open = frame.Parent:WaitForChild("Open")
local baseid = "http://www.roblox.com/asset/?id="

open.MouseButton1Down:connect(function()
    frame.Visible = not frame.Visible
end)

confirm.MouseButton1Down:connect(function()
        local decal = tonumber(tostring(input.Text))
        open.Image = baseid..tostring(decal-1)
        input.Text = tostring(decal)
end)