vertical alignment error

[Marc Hohl]

Hello list,

I tried to simplify the make-...-bar-line functions
recently defined in scm/bar-line.scm.

However, if I replace the definition

(define (make-simple-bar-line grob extent thickness)
  (let ((blot (if rounded
                  (layout-blot-diameter grob)
                  0)))

        (ly:round-filled-box (cons 0 thickness)
                             extent
                             blot)))

with

(define (make-simple-bar-line grob extent thickness)
  (let* ((extent (ly:grob-property grob 'bar-extent))
        (blot (if rounded
                  (layout-blot-diameter grob)
                  0)))

        (ly:round-filled-box (cons 0 thickness)
                             extent
                             blot)))

(I know that the argument 'extent' is redundant; I'll try to
reduce all arguments to 'grob' and calculate the rest from
inside the rooutine to make extensions easier...)

then I get:

Programmierfehler: vertical alignment called before line breaking
Fortsetzung, die Daumen drücken

when I try to compile a score with a span bar:

\score {
  \new StaffGroup <<
     \new Staff {
       \new Voice {
         c c c c
       }
     }
     \new Staff {
       \new Voice {
         c c c c
       }
     }
  >>
}

Can someone explain to me why this happens?

TIA,

Marc