How can I make a raycast to a nil point?

It's a confusing question, I know, but I've tried to deal in raycasting (something Kenetic should add as a tag), and I've found that you can only fire at an existing brick and not the skybox, or else the script breaks. How do I do a workaround? I've tried just replacing the mouse.Hit.p with 999, but it was as if I had done nothing to it. It still broke after I shot at the skybox.

Here's my code:

function drawTrail (mag,position,ry)
    local trail = Instance.new ('Part', Workspace);
    trail.formFactor = Enum.FormFactor.Custom
    if mouse.Hit.p ~= nil then 
    trail.Size =  Vector3.new(0.1, 0.1, mag)
    trail.CFrame = CFrame.new ((script.Parent.Handle.CFrame.p + mouse.Hit.p)/2, mouse.Hit.p)
    else trail.Size = Vector3.new (.1,.1,999)
        trail.CFrame = CFrame.new ((script.Parent.Handle.CFrame.p + Vector3.new (999,999,999))/2, Vector3.new (999,999,999))
    end
    trail.BrickColor = BrickColor.new ('Mid grey')
    trail.Transparency = .5 
    trail.Anchored = true
    trail.CanCollide = false
    trail.TopSurface = Enum.SurfaceType.Smooth
    trail.BottomSurface = Enum.SurfaceType.Smooth
    for i = .5,1,.1 do
        trail.Transparency = i
        wait (.1)
    end
    game.Debris:AddItem (trail, .1)
end