ly:grob-system not working as expected

[Mark Polesky]

Compiling this file displays "#<Grob System >" on the stdout:


{
  \tweak before-line-breaking
    #(lambda (grob)
       (let* ((NoteColumn  (ly:grob-parent grob X)) 
              (PaperColumn (ly:grob-parent NoteColumn X)) 
              (System      (ly:grob-parent PaperColumn X)))
       (display System)))
  c'' 
}

I expected this to do the same, but instead it prints "()".

{
  \tweak before-line-breaking
    #(lambda (grob) (display (ly:grob-system grob)))
  c'' 
}

Am I missing something?  Or is ly:grob-system broken?

Thanks.
- Mark