half diminished chord

[Stefan Thomas]

Dear Lilypond-users,
in the below quoted example the symbol for the half diminished chord is, in my opinion, not printed correctly. 
How can I get the right one?
\version "2.11.59"
\header { title ="Beautiful love" composer = "Victor Young" }
melodie = { \key d \minor \time 4/4 \partial 2. 
    d'4 e' f'     
        a' 2. g'4 
    f'4 . e'8 d'4 e'8 f' ~
    f'1 ~
    f'8 f'4. g'4 a' 
    c'' 2. bes' 4
    a'4. g'8 f'4 g'8 a' }
           akkorde = \chordmode { e2.: e1:m7.5- %% this should be the half diminished diminished chord, but it comes out wrong 
            a1:7 } 
            \score {
  <<
    \new ChordNames {
      \set chordChanges = ##t
      \akkorde
    }
    \new Staff \melodie
  >>  
  \layout{ }
}