convert-ly V1.6.10 to V2.2.0 midi tranposition ?

[Patrick Stanistreet]

I have been using  version 1.6.10  till recently but am now converting
all my ly files to version 2.2.0 with convert-ly
I was using this form to lower the midi pitch of the guitar an  octave

     \property Staff.midiInstrument = #"acoustic guitar (nylon)"
     \property Staff.instrument = "guitar"
     \property Staff.transposing = -12

convert-ly  produces

     \set Staff.midiInstrument = #"acoustic guitar (nylon)"
     \set Staff.instrument = "guitar"
     \set Staff.transposing = -12

But when I run it through  lilypond -m  I get an error message

Parsing...
Interpreting music... warning: Can't find property type-check for 
`transposing' (translation-type?).  Perhaps you made a typing error?
warning: Doing assignment anyway.