Error in Extending Lilypond example

Saul Tobin <[email protected]>
Newsgroups gmane.comp.gnu.lilypond.general,gmane.comp.gnu.lilypond.bugs
Message-ID <CAPF+3FGHRJajxYj4TbZTWf9YOEFEveXTtRBNN1dfFQEw-iNF8g@mail.gmail.com> 
The fourth example engraver here:
https://extending-lilypond.readthedocs.io/en/latest/extending/translation.html#fourth-engraver-example

Running this code in 2.24 crashes on initializing the engraver with In
procedure ly:spanner-set-bound!: Wrong type argument in position 3
(expecting Item): ().
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