Book with bookpart stored in variable: expecting real number

[Caagr98]

This code:

⋘
\version "2.19.81"
bk = \book{\bookpart{\score{a}}}
\bk
⋙

gives a strange error:

⋘
GNU LilyPond 2.19.81
Processing `bpart.ly'
Parsing.../usr/share/lilypond/2.19.81/scm/lily-library.scm:243:5: In procedure 
ly:book-process in expression (process-procedure book paper ...):
/usr/share/lilypond/2.19.81/scm/lily-library.scm:243:5: Wrong type (expecting 
real number): #<undefined>
⋙.

Inserting the \book{...} directly without a variable works as it should.
Replacing the \bk with $bk acts exactly the same.
#bk gives no error, but is instead silently ignored.
$((lambda () bk)) and #((lambda () bk)) act the same as their respective #bk or 
$bk variants.
The #(print-book-with-defaults) function works, but is quite clunky.

Does anyone know why all these methods fail, and what is the proper way to 
store a book in a variable?