Using a Scheme function that returns a bookpart?

[Cameron Hall]

In this example, I have function called "scorePart" that returns a
staff that I use to make parts in the score, and this works. I have
another function called "standalonePart" that returns a bookpart, which
I'd like to use to print individual instrument parts. However, when I
attempt to use it I get an error complaining about bad expression type.
What is going on here that it will accept a staff but not a bookpart?

Thanks,
Cameron

%%%
\version "2.24.4"
\header { title = "Example" }

flute.name = "Flute"
flute.key = c'
flute.music = \fixed c'' { \key c \major c4 d e d c1 }

trumpet.name = "B♭ Trumpet"
trumpet.key = bes
trumpet.music = \fixed c' { \key c \major c4 d c b, c1 }

scorePart =
#(define-scheme-function (instrument) (alist?)
  #{
    \new Staff
    \with { instrumentName = #(ly:assoc-get 'name instrument) }
    { #(ly:assoc-get 'music instrument) }
  #}
)

standalonePart =
#(define-scheme-function (instrument) (alist?)
  #{
    \bookpart {
      \header { instrument = #(ly:assoc-get 'name instrument) }
      \score {
        \new Staff
        \transpose #(ly:assoc-get 'key instrument) c'
        #(ly:assoc-get 'music instrument)
      }
    }
  #}
)

% Score book
\book {
  \header { instrument = "Concert Pitch Score" }
  \score {
    <<
      \scorePart \flute
      \scorePart \trumpet
    >>
  }
}

% Book containing all parts
\book {
  \standalonePart \flute
  \standalonePart \trumpet
}
%%%