item t:number (line thickness) in fret diagrams leads to scheme error

[pls]

Hey all,

the item t:number in fret diagrams (usage described here: 
http://lilypond.org/doc/v2.15/Documentation/notation/instrument-specific-markup)
 leads to the following error:

.../Applications/LilyPond.app/Contents/Resources/share/lilypond/current/scm/fret-diagrams.scm:852:37:
 In procedure cadr in expression (cadr this-list):
/Applications/LilyPond.app/Contents/Resources/share/lilypond/current/scm/fret-diagrams.scm:852:37:
 Wrong type (expecting pair): ()

Tiny example:

\version "2.15.37" % on Mac OS X
\score {
\new Staff {
  \clef "treble_8"
  <c e g c' e'>1^\markup {
      \fret-diagram #"t:1.0;6-x;5-3;4-2;3-o;2-1;1-o;"
    }
  }
}