What's the deal with this "programming error"?

[David Kastrup]

When compiling

\new Voice \with {\consists "Ambitus_engraver"}
{<<\new Voice { c } s>> { c } }

I get the following two "programming error"s.  What's up?

lilypond /tmp/junk.ly
GNU LilyPond 2.13.23
Processing `/tmp/junk.ly'
Parsing...
/tmp/junk.ly:0: warning: no \version statement found, please add

\version "2.13.23"

for future compatibility
Interpreting music... 
Preprocessing graphical objects...
programming error: note column without heads and stem
continuing, cross fingers
/tmp/junk.ly:2:26: programming error: note-column has no direction
{<<\new Voice { c } s>> { 
                          c } }



-- 
David Kastrup