attempt to concatenate local 'Variable' (a nil value) / Error Message?

Well I've made a script, where the Server sends the time of lighting to Client. The Clientside script seems to work fine, but a error in the Output pops up the whole time.

22:06:24.160 - Players.bbudder.PlayerGui.CS.Main.TimeModule [FE]:6: attempt to concatenate local 'STime' (a nil value)

22:06:24.162 - Stack Begin

22:06:24.163 - Script 'Players.bbudder.PlayerGui.CS.Main.TimeModule [FE]', Line 6

22:06:24.164 - Stack End

Clientside:

local CoreGui = game.Players.LocalPlayer.PlayerGui
local Time = CoreGui.CS.Main.TimeDisplay
local Event = game.ReplicatedStorage.ServerTime

Event.OnClientEvent:Connect(function(STime)
Time.Text = ""..STime
end)

Serverside:

local Event = game.ReplicatedStorage.ServerTime
while true do
	wait(1)
	local STime = game.Lighting.TimeOfDay
	Event:FireAllClients(STime)
end

Any answers would be appreciated. How can I avoid this?

++EDIT Even if I replace / Time.Text = ""..STime / with /Time.Text = STime / then this message pops up:

22:23:16.810 - Players.bbudder.PlayerGui.CS.Main.TimeModule [FE]:6: bad argument #3 to > 'Text' (string expected, got nil)

So THEY did fixed my problem about the Error using tostring. But the problem is that the CLIENT SIDE script is firing, EVEN if the SERVER is not firing.

So yes, using tostring fixed the error stuff.