midiInstrument "choir aahs" changes to piano in 2.13.61

[Tony Towers]

LilyPond 2.13.61 seems to have introduced an error in the MIDI output,
where if an instrument is set to "choir aahs", after one note the output
switches to "piano".

The problem does not occur in 2.13.60

---BEGIN---
\version "2.13.60"
\include "english.ly"

\score {
        \new Staff = vocal \with {midiInstrument="choir aahs"} {
                \relative c' { r2 f4 d8 d }
        }
        \midi { }
}
---END---