custom stringTuning not working

[Federico Bruni]

It's the first time I use the new command \stringTuning and I can't get 
it work.
In the following example I should see 0s from the 6th to the 1st string.
What I'm doing wrong?

\version "2.15.41"

music =  \relative c {
  d,8 bes' d g bes d
}

\score {
  \new StaffGroup <<
    \new Staff <<
      \context Voice { \clef "G_8" \music }
    >>
    \new TabStaff  <<
      \set TabStaff.stringTunings = \stringTuning <d, bes' d' g' bes'' d''>
      \context TabVoice { \clef "moderntab" \music }
    >>
  >>
}