Health is not valid member of Model?

Yes, I know what you're thinking; "This is so easy, health is not a valid member of a model!".

But, I used the FindFirstChild() function, but still, it doesn't work.

Client script: (Local script, inside tool)

tool = script.Parent
player = game.Players.LocalPlayer


tool.Equipped:Connect(function(mouse)
    mouse.Icon = "http://www.roblox.com/asset/?id=2312592107"
    mouse.Button1Down:Connect(function()
    game.Workspace.KillExplosion.KillExplosionCreate:FireServer(mouse.Target, tool.Handle)


    end)
end)

Server script: (Normal script, inside workspace)

debounce = false

    script.KillExplosionCreate.OnServerEvent:Connect(function(player, target, tool) 
        if debounce == false and target.Parent:FindFirstChild("Humanoid") then
            debounce = true
            tool.BrickColor = BrickColor.new("Really red")
            player = target.Parent.Humanoid
            player.Health = player.Health - 18
            wait(1)
            debounce = false
            tool.BrickColor = BrickColor.new("Sea green")

    end
end)

I have no idea why this doesn't work. So, when I click on something that doesn't have a Humanoid I get the error:

Health is not valid member of Model

BUT I USED THE FIND FIRST CHILD FUNCTION! WHY??!!

ERROR AT LINE 8, SERVER SCRIPT