Mensural music and midi

[Phil Holmes]

With this code:

\score {
 <<
   \new MensuralVoice = "discantus" \transpose c c' {
     \override Score.BarNumber #'transparent = ##t {
       c'1
     }
   }
   \new Lyrics \lyricsto "discantus" {
     La
   }
 >>
 \layout {}
 \midi {}
}

(partly taken from the NR) I get:

warning: cannot find or create new `MensuralVoice'
Mensural.ly:8:16: warning: cannot find Voice `discantus'

   \new Lyrics 
               \lyricsto "discantus" {

Take the midi {} out and it disappears.  Bug?
--
Phil Holmes