problem with crescendo-text

[Stefan Thomas]

Dear Lilypond-users,
in the below quoted example I can't see the "crescendo" and I don't know why. Does someone know a solution?

\version "2.11.49"
{
\set crescendoSpanner = #'text
\override DynamicTextSpanner #'dash-period = #-1.0
     a'2\< a a a\mf }