Why is this RemoteEvent using Player as a parameter when I give it a string as an argument?

Output:

sub is not a valid member of Player

Hierarchy:

Workspace
	HoloScript (Script)
		CustomChatScript (LocalScript)
		Chatted (RemoteEvent)

I'll include all the code that I think you will need to help me solve this problem. Let me know if you need more. I apologize for all the reading.

In the past, I used the Roblox Chatted event to fire commands, but since some custom chat systems remove the Roblox chat bar, I had to make some changes. What I don't understand is why I'm getting this error, since I'm never giving an argument equal to Player.

Holo script:

local cmd = "start"
local cmdNum = cmd:len()
local customChat = true

function onChat(msg)
	if msg:sub(1,cmdNum) == cmd then --This is the line that's causing the error 
	--stuff
	end
end

game.Players.PlayerAdded:connect(function (player)

	repeat wait() until player.Character --Helps prevent glitching
	
	if isAdmin(player) then --Defined earlier; works fine.
		if customChat then
			script.Chatted.OnServerEvent:connect(onChat)
			
			local clone = script.CustomChatScript:Clone()
			clone.Parent = player:WaitForChild("PlayerGui")
			
			player.CharacterAdded:connect(function(chr) 
				local clone = script.CustomChatScript:Clone()
				clone.Parent = player:WaitForChild("PlayerGui")
			end)
			
		else --Otherwise,
			player.Chatted:connect(onChat) 
		end
	end
end)

Here's the LocalScript named CustomChatScript, located in workspace.HoloScript:

local plr = game.Players.LocalPlayer
	repeat wait() until plr.Character
local chr = plr.Character
local mouse = plr:GetMouse()

local startCommand = "/" 

local addToString = false
local theString = ""

game.StarterGui:SetCoreGuiEnabled("Chat", false)

mouse.KeyUp:connect(function(key)
    if key == startCommand and not addToString then
        addToString = true 
		chr:WaitForChild("Humanoid").WalkSpeed = 0
    elseif key:byte() == 13 and addToString then --13 = "Enter"
		workspace:WaitForChild("HoloScript").Chatted:FireServer(theString)--See? I'm not feeding in a player...
        theString = ""
        addToString = false
		chr:WaitForChild("Humanoid").WalkSpeed = 16
    elseif key:byte() ~= 13 and addToString then
        theString = theString..key
    end
end)