Strange behaviour with unknown alternativeNumberingStyle

[Thomas Morley]

Hi all,

please have a look at:

\version "2.25.0"

{
  \override Score.BarNumber.break-visibility = ##(#f #t #t)
%  \set Score.alternativeNumberingStyle = #'whatever
  b1
  \repeat volta 2 { c' c' }
  \alternative { d' e' }
  f'
}

As soon as the style-setting is uncommented it behaves like
  \set Score.alternativeNumberingStyle = #'number
shouldn't an unknown style behave like the default '()?

tl;dr
This probably prevents me defining a knew style with a custom engraver
as fix for lsr-snippet 1080 "Incrementing bar numbers in volta
repeats"
https://lsr.di.unimi.it/LSR/Item?id=1080

Cheers,
  Harm