midi-instrument problem

[Till Rettig]

Hello,

does somebody have any idea, why I get only drum set as a midi 
instrument for the following score:

\version "2.6.3"
\include "deutsch.ly"

alt = {\relative c''
{
\time 4/4
\set Staff.midiInstrument = "choir aahs"
%\key g \minor
\clef "treble"
a2 c e1 d4 c h a
}}
tenor = {
\time 4/4
\set Staff.midiInstrument = "choir aahs"
%\key g  \minor
   % main
   \clef "treble_8"
e2 c a1 h4 c d e
}

\score{
\context StaffGroup = choirStaff <<
\context Voice = alt << \alt>>
\context Voice = tenor <<  \tenor >>
>>
\midi {\tempo 4=60 }
\layout {
}
}

It doesn't happen, when I don't make this predefinition of the voices 
but insert one after the other into the \score directly via \new Staff 
-command. But I actually would like to keep this definition as it makes 
files much clearer.

Thank you

Till Rettig