Harp midi export

[LeGrosMario]

Hi

I'm trying to export a midi file using the harp instrument.
Orhestral harp is listed as #47 midi instrument,
http://www.midi.org/techspecs/gm1sound.php .
Unfortunately I get an error when I state this intrument in my lilypond source
score. I can use some other intruments (listed in the lily doc), but I need an
orchestral harp.
NB: my sf2 file has a pretty realistic acoustic harp samples set.

Any idea please ?

\set Staff.midiInstrument = #"orchestral strings" works but \set
Staff.midiInstrument = #"orchestral harp" fails. I tried #"47', which did not
help.

Thanks.

(Btw: I'm totally new to lilypond and I have to say this piece of software is
*amazing*, and so is the documentation.)