Page reference problem

[Jacques Menu]

Hello folks,

The simple example in the doc works fine, but what if the score contains 
parallel music? An extra staff is printed in my attempts.

Thanks for the help!

JM

\version "2.23.80"

% disposition
% ----------------------------------------------------------------------

\header {
  title                = "Kumbaya"
  composer             = "Negro-spiritual"
}

\paper {
  print-page-number = ##t
  print-first-page-number = ##f

  oddHeaderMarkup = \markup {
    DenierePage à la page \page-ref #'DenierePage "0" "?"
  }
  %  evenHeaderMarkup = \oddHeaderMarkup
  evenHeaderMarkup = \markup {
    \unless \on-first-page
    \fill-line {
      \line {
        \fromproperty #'header:title
        " - Page "
        \concat {
          \fromproperty #'page:page-number-string
          "/"
          \page-ref #'DenierePage "0" "?"
          % "0": un chiffre suffit pour indiquer la largeur du chanmp à prévoir
          % "2' ou "3": devrait pouvoir être "?"
        }
      }
      \line {
        DenierePage à la page \page-ref #'DenierePage "0" "?"
      }
    }
  }
}

\layout {
  \context {
    \Score
    % autoBeaming = ##f % to display tuplets brackets
  }

  \context {
    \Voice
  }
}


\book {
  \score {
    {
      <<
        a1
        b1
      >>

      \label #'DenierePage %%%%%%%%%%% HERE
    }
  }
}