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)