Bad expression error with scheme function returning bookpart

[Gianmaria Lari]

 #In the following code each call to \myScore
# should returns a couple of scores (one midi
# score and one layout score) but
# it doesn't compile and return this error:
# document.ly:9:1: error: bad expression type
#
#\myScore {\repeat percent 2 {c' d' e' f'}}

\version "2.19.81"
myScore = #(define-scheme-function (music) (ly:music?) #{
  \bookpart {
    \score { $music \layout{} }
    \score { \unfoldRepeats $music \midi{}}
  } #} )

\myScore {\repeat percent 2 {c' d' e' f'}}
\myScore {\repeat percent 2 {g c' c' c'}}