OnServer Event attempt to concatenate local what?

I'm working on a Command which plays music globally and Roblox keeps outputting this following output.

ServerScriptService.EventFunctions:72: attempt to concatenate local 'AudioId' (a userdata value)

the code is as follows:

On Server Script

--// Local Variables
local PlayMusicEvent = game:GetService("ReplicatedStorage"):WaitForChild("PlayMusic")

--// OnServerEvent
PlayMusicEvent.OnServerEvent:Connect(function(AudioId)

    local S = Instance.new("Sound")
    S.Playing = true
    S.Volume = 1
    S.SoundId = "rbxassetid://" .. AudioId
    S.Name = "AddedMusic"
    S.Parent = game.Workspace

end)

On Client LocalScript

--// Local Variables
local Event = game:GetService("ReplicatedStorage"):WaitForChild("PlayMusic")
local AudioIDField = script.Parent.Parent.MusicIdFieldFrame.MusicIdField

--// FireServer
game.ReplicatedStorage.PlayMusic:FireServer(AudioIDField.Text)

I haven't a clue why it keeps saying that local 'AudioId' is a userdata value whenever the Event is fired is a really straight forward script pass the ID to the server to then create the Sound Object and put the SoundId the user picked. Can anyone else spot why this happens?