stem direction programming error

[Kieren MacMillan]

Hello all,

I've got the following code in a solo violin score:

    r4 << { \voiceOne b'( c) } \new Voice { \voiceTwo <d,! \tweak  
#'duration-log #4 g,>2 } >> \oneVoice <ef \tweak #'duration-log #4 g,  
d''>2\downbow \typesetOff <a ef f'>2 ~   | % 3
    <a f'>4\mf ef' \times 2/3 { d4\tenuto af bf } \typesetOn b8 e,! 
\> b! bf   | % 4

where \typesetOff and \typesetOn are shorthand for \set-ting the  
Score.skipTypesetting property.

When I allow the non-typeset blob

    <a ef f'>2 ~ <a f'>4\mf ef' \times 2/3 { d4\tenuto af bf }

to be typeset [i.e., remove the \typeset commands], I get

    Preprocessing graphical objects...
    programming error: Stem dir must be up or down.
    continuing, cross fingers

and then Lilypond fails (i.e., Lilypad simply brings up the  
previously-typeset version of the PDF file).

Unfortunately, I can't seem to duplicate the behaviour in a minimal  
file, but I was hoping someone could look at this code and tell me  
what could possibly be wrong with the little excerpt that's causing  
the problem. [I'm using 2.12.2 and 2.13.3 on Mac OS X 10.4 — both  
versions fail while trying to typeset the chunk.]

Thanks!
Kieren.