markup in textspanner doens't work well

[Stefan Thomas]

Dear community,
in the below quoted snippet, the markup in the textspanner doesn't work as expected.
I can't see a sign for a quarter-note!
How can I produce it?
\version "2.12.2"
{ 
 \override Staff.TextSpanner  #'(bound-details left text) = #"rit." 
  \override Staff.TextSpanner  #'(bound-details right text) = \markup {   \note #"4" #1   "=60" } 
  \time 4/4 
  \clef bass c4 \startTextSpan c e f 
  g1\stopTextSpan
}