How to use RemoteFunctions?

Here's my scenario: each player has 500 health and 500 "shield" which is essentially extra health. I've assigned shield to an IntValue inside a GUI and I want shield to be depleted before health. I created a RemoteFunction for defining damage to players and I also have a brick that is supposed to damage players when touched.

Here is the LocalScript (inserted inside a ScreenGUI):

--Remote functions
local damageInfantry = game.ReplicatedStorage.DamageInfantry
function damageInfantry.OnClientInvoke(player, humanoid, amount)
	if shield.Value > 0 then
		shield.Value = shield.Value - amount
		if shield.Value < 0 then --More damage done than shield remaining
			local difference = shield.Value * (-1)
			humanoid:TakeDamage(difference)
		end
	else
		humanoid:TakeDamage(amount)
	end
end

Here is the server script (inserted inside the brick):

--Functions
function onTouch(part)
	local character = part.Parent
	local humanoid = character:FindFirstChild("Humanoid")
	if humanoid then
		if canDamage then
			damageInfantry:InvokeClient(humanoid, 100)
			canDamage = false
			wait(1)
			canDamage = true
		end	
	end
end

This gives me an error: attempt to perform arithmetic on local 'amount' (a nil value) What am I doing wrong and is there a better way to handle damaging humanoids with this "shield" mechanic?

Thanks in advance.

function damageInfantry.OnClientInvoke(player, humanoid, amount)

You have an extra argument here, it should only be humanoid, amount. Usually, the event only takes the player argument when it's from client to server so that the server knows who sent it.

function damageInfantry.OnClientInvoke(humanoid, amount)