trouble with \set

[David Bobroff]

I'm having trouble with \set tupletSpannerDuration.  Am I doing
something wrong or have I found a bug?  I need to make numerous changes
to tupletSpannerDuration during a piece.  After doing \set, when I need
to change it, should I do \unset and then \set to the new setting?  I'm
getting "programming error" messages.  This piece of LilyPond code
produces the error message on my setup.  The output looks fine.  

\score {
    \notes \relative c' {
        \set tupletSpannerDuration = #(ly:make-moment 1 4)
        \times 2/3 {c8 c c c c c c c c c c c}
        \set tupletSpannerDuration = #(ly:make-moment 1 2)
        \times 2/3 {c4 c c c c c}
 }
    \paper { }
}

The console says:

lilypond (GNU LilyPond) 2.1.29
Running usr...
Now processing: `/home/lily-puts/fennelly/sasquatch/test.ly'
Parsing...Interpreting music...programming error: Skipped something?
Grob TupletBracket ended before I expected it to end. (Continuing;
crossing fingers)
programming error: Skipped something?
Grob TupletBracket ended before I expected it to end. (Continuing;
crossing fingers)
[3]
Preprocessing graphical objects... 
Calculating line breaks... 
paper output to `test.tex'...