How does \musicMap not return music?

[Simon Albrecht]

Hello,

surely I overlooked something:

I’m writing a function to truncate MMRs for rehearsal MIDI. This is the 
code so far:

%%%%%%%%%%%%%
\version "2.19.53"

truncateRests =
#(define-music-function (mus) (ly:music?)
   (let ((trunc-rests (lambda (e) (if (music-is-of-type? e 
'multi-measure-rest-event)
                                     (ly:music-set-property! e 
'duration (ly:make-duration -1))))))
     (musicMap trunc-rests mus)))

\truncateRests { \time 4/2 R\breve*10 }
%%%%%%%%%%%%%%

Now Lily complains on that, saying ‘document.ly:9:1: error: music 
function cannot return ##<unspecified>’. I’m puzzled – musicMap is 
itself a music function, so how could truncateRests not return music?

TIA, Simon