Devnull: how to make it take no space

[Jean-Charles Malahieude]

Hi all,

In the book I'm engraving, it happens that some instruments get "tacet" 
for certain pieces. In order to generate a full table of contents, I 
treat them through "Devnull". The problem is that I often get what follows.

In this tiny example, I would like the "devnull" to not take any 
vertical space. I've played with the various variables, but with no success.

---8<---
\version "2.14.0"
\score {
  \repeat unfold 4 { c'1 \break }
  \header { piece = "Piece one" }
}
\score {
  \new Devnull { s4 }
  \header { piece = "Piece two" opus = "TACET." }
}
\score {
  \repeat unfold 9 { e'1 \break }
  \header { piece = "Piece three" }
}
--->8---

Thanks for your help,
Jean-Charles