scheme function doesn't work anymore

[VSD]

I was successfully using the following simple scheme function in all the  
2.5.x series until 2.5.29-1 (all win native under WXP):

ChordOffset = #(def-music-function (parser location offset) (pair?)
    #{
        \override ChordNames.ChordName #'extra-offset = #$offset
    #}
)

mychords = \chordmode
{
        \ChordOffset #'(-0.75 . 0)

        ges2:maj7.4+ des2:9 ...
}

but in version 2.5.29-2 this doesn't work anymore. The log file is:

# -*-compilation-*-
Changing working directory to `C:/music/tst'
Processing `C:/music/kk.ly'
Parsing...


... and I don't get any output file. If I replace the function call by the  
corresponding lilypond code:

\override ChordNames.ChordName #'extra-offset = #'(-0.75 . 0)

then it works as expected.

Is this a bug or am I doing something wrong?

Thanks and greetings,

Vincent