Problem with transposition

[fazer666]

Hi,

I'm trying to write some music for Tenor Sax (Bb). I have melody and chords 
above the staff. When I put "\transposition bes" directive, melody is 
correct in midi file, but not chords.

Here is a snippet of my code :

\score 
{
  "double_opening_chevrons"
    \chords
    {
      \transpo
      gis,1:m7.5- cis:7
      fis,1:m7.5- b,:7.9-
      e,1:m7.5- cis2:m7.5- fis:7
      b,1:m6 c:9
    }

    \new Staff \relative g'
    {
      b8 d fis2~ fis8 gis | f2 cis' |
      a,8 c e2~ e8 fis | ees2 b' |
      g,8 bes d2~ d8 e | cis2~ cis8 bes a g |
      fis2. fis8 g | c,1 |

    }
  "double_closing_chevrons"

NB: I wrote "double_opening_chevrons" because with real characters, gmane 
though I was top-posting...
 
I tried to change the position of the directive (for example put only one 
directive befor the \chords directive) with no success.

Any idea ?
Thanks in advance for your help