Lyrics disappear with associatedVoice in a duet

[Eric van Gyzen]

Fellow Lilypond Users:

I'm writing a vocal duet on a ChoirStaff with lyrics between two staves. 
 When one voice rests, I'm trying to use associatedVoice to make the 
lyrics follow the other voice.  I would expect the lyrics to remain 
between the staves, but the lyrics disappear completely.

Here is the tiniest example I can write:

========
\version "2.16.0"  % also tried 2.17.6

\new ChoirStaff <<
    \new Staff \new Voice = "SopranoVoice" \relative c' { c4 d e f r1 }
    \new Lyrics \lyricsto "SopranoVoice" {
        So -- pran -- os rule.
        \set associatedVoice = #"AltoVoice"
        Al -- tos are cool.
    }
    \new Staff \new Voice = "AltoVoice" \relative c' { r1 f4 e d c }
>>
========

It looks like this:

        http://vangyzen.net/music/associatedVoice_problem.pdf

I would like the Alto's lyrics above her notes.

What am I doing wrong?

Thanks for any help you can offer.

--Eric  (near Minneapolis, Minnesota, US)