Lyrics provoke "Moment is not increasing" complaint

[Roman Stawski]

I want to have one group of parallel staves following another
sequentially. The constitution of the two groups is quite different.
(In the real piece I'm using \removeEmptyStaffContext to hide the
first group when I finish with it.) A cut-down example is:

- - - -[START]
\version "2.11.60"
\layout { ragged-right = ##t }

example = {
  <<
    \new Staff { \new Voice = "a" \relative c'' { a1 } }
    %\new Lyrics { \lyricsto "a" \lyricmode { Doo } }
  >>
  <<
    \new Staff { \new Voice = "c" \relative c' { c1 } }
    \new Lyrics { \lyricsto "c" \lyricmode { dah } }
  >>
}

\score { \example }
- - - -[END]

This snippet works, but when I uncomment the Lyrics line, I get

- - - -
...
Interpreting music...
programming error: Moment is not increasing. Aborting interpretation.
continuing, cross fingers
test.ly:11:18: warning: cannot find Voice 'c'

    \new Lyrics {
                  \lyricsto "c" \lyricmode { dah } }

...
- - - -

and the second group is corrupted in the display.

The only reference to this problem that I could find was a posting
from Morten Lemvigh last year:
http://thread.gmane.org/gmane.comp.gnu.lilypond.general/32409/focus=32412
He solves the problem by changing the lyrics, which isn't an option in
my case.

So what am I dong wrong? Does anyone have any ideas how to solve/get
around it?

Thanks

Roman