\bookpart question

[Jacques Menu]

Hello folks,

I naturally tend to specify a number of pages in a bookpart this way:

  \bookpart {
    \layout {
      page-count = 1
    }
    \new Staff = "HautboisDeux"
    \with {
      instrumentName = "Hautbois 2"
      shortInstrumentName = "Hbs 2"
    }
    <<
      \context Voice = "HautboisDeux" <<
        \HautboisDeux
      >>
    >>
  }

What is the logic behind that being rejected, and the need to use a \paper 
block instead, which does what I want actually?

 Starting lilypond 2.19.83 [InNativitatemDomini.ly]...
Processing 
`/Users/menu/Documents/LaTeX/PartitionsLilypond/Courvoisier_Jean_Daniel/BaroccOpera/InNativitatemDomini.ly'
Parsing...
/Users/menu/Documents/LaTeX/PartitionsLilypond/Courvoisier_Jean_Daniel/BaroccOpera/InNativitatemDomini.ly:2625:5:
 error: need \paper for paper block
    
    \layout {
/Users/menu/Documents/LaTeX/PartitionsLilypond/Courvoisier_Jean_Daniel/BaroccOpera/InNativitatemDomini.ly:2641:5:
 error: need \paper for paper block
… … … … … … 
 
Thanks for your help!

JM