Problem with lyrics

[Famille Meigniez]

You can find attached a typical example of a song with a 'refrain' and same verses. But I don't 
understand why Lilypond shows me the following error message during the processing:

GNU LilyPond 2.6.3
Processing `SoisTemoin_2.ly'
Analyse...
Interprétation en cours de la musique...[8]
erreur de programmation: moving backwards in time
poursuite; croisons les doigts
[16][24][25]
Pré-traitement des éléments graphiques...
Calcul des bris de lignes... [3][6][9][12][15][18][21][24][26]
Calculating page breaks...
Layout output to `SoisTemoin_2.ps'...
Conversion à « SoisTemoin_2.pdf »...

and why it adds two empty bars between the 'refrain' and the verses as you can 
see in the pdf file.

Could you explain me what I do wrong or... if it is, maybe, a bug ?

Thanks in advance
P. Meigniez

P.S.: To have a 'correct' printout, I had to add artificially a note in the 'refrain'. By example, 
by replacing the last rest by a c.

LyricsProblem.zip
Description: Zip archive