Chat script cannont concantenate local 'msg'?

Basically, I made a chat. On the main script, an error occurs on line 20, "attempt to concantenate local 'msg' a nil value" I have no idea whats wrong with it. Nor do I know what this means...

ServerScript:

function UpdateOldLabels(Parent)
    for i,v in pairs (Parent:GetChildren()) do
        if v.Name:sub(1,4):lower() == "line" then
            local LineNumber = v.Name:sub(5)
            if LineNumber == 12 then
                v:Destroy()
            else
                v.Name = "line" .. tostring(tonumber(LineNumber) + 1)
                v.Position = v.Position - UDim2.new(0, 0, 0.035, 0)
            end
        end
    end 
end




function NewLine(parent, plr, msg)
    local New = Instance.new("TextLabel", parent)
    New.Text = plr.Name .. ": " .. msg
    New.Size = UDim2.new(0.35, 0, 0.026, 0)
    New.Position = UDim2.new(0, 0, 0.315, 0)
    New.BorderSizePixel = 0
    New.BackgroundTransparency = 1
    New.TextColor3 = Color3.fromRGB(255, 255, 255)
    New.TextStrokeTransparency = 1
    New.FontSize = "Size10"
    New.TextXAlignment = "Left"
    New.TextYAlignment = "Top"
    New.Name = "Line1"
    New.ClipsDescendants = true
end
workspace.Events.Other.Chat.OnServerEvent:connect(function(plr, msg)
        for _,v in ipairs(game:GetService("Players"):GetChildren()) do
            NewLine(v:WaitForChild("PlayerGui").Chat.Container.ChatLines, plr, msg)
            UpdateOldLabels(v:WaitForChild("PlayerGui").Chat.Container.ChatLines)
        end
end)

LocalScript:

script.Parent.InputEnded:connect(function()
    game.Workspace.Events.Other.Chat:FireServer(game.Players.LocalPlayer, script.Parent.Text)
    script.Parent.Text = [[Click here or press the "/" key to chat.]]

end)