Why does the output reads "attempt to concatenate a nil value"?

So I have a function that concatenates two values together, a parameter and a GUI's text. The problem is the script attempts to concatenate a nil value. For some reason I changed the GUI's text before calling the function and it still prints it.

I'm not sure why this happens, I don't think it's a bug though but more of the syntax?4

Here is a little snippet from the script:

--Timer() takes a number and concatenates it with a GUI's Text. 
local function Timer(formula)

    if formula == nil then
        TimerGui.Enabled = false
    else
        TimerGui.Enabled = true
        TimerText.Text = (TimerText.Text..formula) 
    end
end


--Tool activation event:
local function ToolMouseClick(Mouse)
    print("Tool clicked")
    if player.Team ~= game.Teams.Survivors and player.Character.Humanoid.Health ~= (0) then
        Tool:Destroy()
        return
    end
    Timer.Text = ("Flashbang detonating in:  ")
    Timer(1) 
end

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