Interpreter Reports: "Programming Error: Moment Not Increasting"

[Andy Cowan]

I'm trying to lay out a fairly simple score to "Oh Susanna" for a friend, and have received the following utterly unhelpful output from the interpreter.  The resulting PDF includes the first section of the song (which is entered as a discrete block), correctly-engraved.  Then it has a blank measure, then it stops.

The relevent line of the output seems to be : "programming error: Moment is not increasing."  I don't even know what a moment is or why it's supposed to increase!

Can somebody help me to understand what's going on here?

Thanks,

Andy
====

The-Irish-Rover:~ acowan$ lilypond Susanna.ly
GNU LilyPond 2.10.25
Processing `Susanna.ly'
Parsing...
Interpreting music... [8]
programming error: Moment is not increasing. Aborting interpretation.
continuing, cross fingers
Susanna.ly:56:13: warning: cannot find Voice `uniqueContext1'

  \addlyrics 
             {
[11]
Preprocessing graphical objects...
Calculating line breaks... 
Drawing systems... 
Layout output to `Susanna.ps'...
Converting to `Susanna.pdf'...
====
Here's the contents of the .ly file:

% Oh, Susanna
% Key of D Major

#(set-default-paper-size "letter")
#(set-global-staff-size 20)

\paper { system-count = 6 }

Verses = \relative d' {
  \clef treble
  \key d \major
  \time 4/4
  \partial 4 d8 e
  %\addlyrics { I _ }
  <<  
  \repeat volta 2 {
    fis4 a a4. b8
    a4 fis d4. e8
    fis4 fis e d 
    e2. d8 e
    fis4 a a4. b8
    a4 fis d4. e8
    fis4 fis e e
  }
  \alternative {
    { d2. d8e }
    { d1 }
  }
  >>
  \addlyrics {
    come from al a ba ma with my ban jo on my knee.
    and I'm goin' to Lou -- si -- an -- a my __ _ true love for to see. It _
  }
  \addlyrics {
    rained so hard the day I left the wea -- ther it was dry.
    The _ sun so hot, I froze to death, Su -- San -- na don't you _ _ _ cry
  }
}

Refrain = \relative g' {
  
  <<
  \repeat volta 2 {
    
    g2 
    g2 
    b4 b2 b4
    a4 a fis d
    e2. d8 e
    fis4 a a4. b8
    a4 fis d4. e8
    fis4 fis e e
    d1
    }
  \addlyrics {
    Oh, Su -- san -- na, oh don't you cry for me.
    For I come from Al -- a -- ba -- ma my __ _ true love for to see.
  }
  >>
}

\header {
  title = "Oh, Susanna"
  composer = "Stephen Foster, 1848"
}

\new Staff {
  \Verses
 % \break
  \Refrain

}