Snippet 960 fails with Ly 2.25

[K. Blum]

Hello,

LSR snippet 960 works alright with LilyPond 2.24.3 and earlier.
Ly 2.25.1 (and later) aborts with an error message, see below.
In the docs I haven't found any changes to the functions in use.
Also tried convert-ly, but it did not change anything from the code.
Am I missing something or is this a bug?

Snippet 960: https://lsr.di.unimi.it/LSR/Item?id=960

Here is a (not quite) MWE:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
colorSpan =
#(define-music-function (y-lower y-upper color)
   (number? number? color?)
   #{
     \once \override HorizontalBracket.stencil =
     $(lambda (grob)
        (let* (
                (area (ly:horizontal-bracket::print grob))
                (X-ext (ly:stencil-extent area X))
                (Y-ext (ly:stencil-extent area Y)))
          (set! Y-ext (cons y-lower y-upper))
          (ly:grob-set-property! grob 'layer -10)
          (ly:make-stencil (list 'color color
                                 (ly:stencil-expr (ly:round-filled-box
X-ext Y-ext 0))
                                 X-ext Y-ext))))
     \once\override HorizontalBracket.Y-offset = #0
   #})

\score {
  {
    \colorSpan #-4 #4 #green
    c'2\startGroup g' c'\stopGroup r
  }
  \layout {
    \context {
      \Voice
      \consists "Horizontal_bracket_engraver"
    }
  }
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

The error message from Ly 2.25.1 reads:
-----------------------------------
Starte lilypond.exe 2.25.1 [mwe.ly]...
»C:/Users/Flower/AppData/Local/Temp/frescobaldi-0hwxag11/tmp9kixmwyv/mwe.ly«
wird verarbeitet
Analysieren...
Interpretation der Musik...
Vorverarbeitung der grafischen Elemente...
Ideale Seitenanzahl wird gefunden...
Musik wird auf eine Seite angepasst...
Systeme erstellen...
C:/Portable/lilypond-2.25.1/share/lilypond/2.25.1/ly/init.ly:64:2:
Fehler: Guile signaled an error for the expression beginning here
#
 (let ((book-handler (if (defined? 'default-toplevel-book-handler)
In procedure cadddr: Wrong type (expecting pair): ()
Wurde mit dem Return-Code 1 beendet.