Failure of \RemoveEmptyStaves

[obscureed]

> I'm not top-posting.

I'm trying to transcribe songs with piano accompaniment, including a piano
intro.  The melody stave is not needed during the intro, but the
\RemoveEmptyStaves command seems not to remove it:
------------------
\version "2.14.2"
melody = \relative c' { s4 }
upper  = \relative c' { g4 }
\score {
  <<
    \new Voice = "mel" { \melody }
    \new PianoStaff <<
      \new Staff = "upper" \upper
    >>
  >>
  \layout { \context { \RemoveEmptyStaves } }
}
-------------------

The desired behaviour was generated in previous versions -- search for "slumber"
or "Alice" at
http://www.mutopiaproject.org/cgibin/make-table.cgi?Instrument=Voice.  I've
tried running convert-ly on these, and the new output does not follow the old,
desired behaviour.  Interestingly, the new output from slumber.ly (after
convert-ly) correctly removes the melody line from the second line of the intro,
but not the first.  Does the clef/time/key info in the first line now prevent it
from being empty?

Thanks.
Ed, Sheffield, UK.