player Argument must be a Player Object error using FireClient?

I'm using a FireClient remote event to make a GUI pop up for the player being clicked on by another player.

Server Script:

script.Parent.MouseClick:Connect(function(player)
	local playerClickedOn = script.Parent.Parent.Parent
	local guiEvent = game:GetService("ReplicatedStorage"):WaitForChild("GUI").guievent
		guiEvent:FireClient(player,playerClickedOn)
end)

Then, after the player is clicked on by another player, a GUI should pop up for the player who was clicked on.

Local Script:

local guiEvent = game:GetService("ReplicatedStorage"):WaitForChild("GUI").guievent
	guiEvent.OnClientEvent:Connect(function(player,playerClickedOn)
		Player.PlayerGui.YesNo.Enabled = true
		Player.PlayerGui.YesNo.PickUp.Visible = true
		Player.PlayerGui.YesNo.PickUp.TextLabel.Text = player.Name.." has clicked on you"
	end)

However, I get this error:

FireClient: player argument must be a Player object, Line 4

What does this mean and how do I fix this? What I want is for the event where the player that was clicked on has a GUI pop up telling them who clicked on them.