How do you get the center of the players screen?

I am working on an fps that is mobile friendly. By doing this I am trying to make a local script get the center of the players screen with ViewportPointToRay but it doesnt seem to work because I am apparently not sending Vector3 values.

Local Script

mouse.Button1Down:Connect(function()
    local camera = workspace.CurrentCamera
    local viewportPoint = camera.ViewportSize / 2
    local mousePosition= camera:ViewportPointToRay(viewportPoint.X, viewportPoint.Y, 0)
        Shoot:FireServer(mousePosition)
end)

Server Script

local ray = Ray.new(root.Position, (mousePosition.p - root.Position).Unit * 1000) 

Error bad argument #1 to '?' (Vector3 expected, got Ray)