mmrest-oflength not generating MMR 2.15.24

[Paul Scott]

Hi,

Can someone tell me why skip-of-length work in this example 
but mmrest-of-length doesn't?

\version "2.15.24"

\layout {
  \context {
    \Staff
    \remove "Time_signature_engraver"
  }
}

timing = { }

cnotes = \relative c'' { \cadenzaOn c8[ d e f g] \cadenzaOff }
snotes = { $(skip-of-length cnotes) }
mnotes = { $(mmrest-of-length cnotes) }

\score{ 
  <<
    \new Staff \cnotes 
    \new Staff \mnotes
    \new Staff \snotes
  >>
}


TIA

Paul Scott