minimum-length does not work for DynamicTextSpanner

[Frédéric Bron]

I the following snippet, I would like the second c to be shifted to
the right so that there is enough space for the text crescendo.
minimum-length works fine with hairpin crescendo but not with dynamic
text spanner.
What can I do?
Frédéric

\version "2.12.2"
\relative c' {
        \once \set crescendoText = \markup { \italic "cresc." }
        \once \set crescendoSpanner = #'text
        \once \override DynamicTextSpanner #'style = #'dashed-line
        %I tried to add the following line but without success
        %\once \override DynamicTextSpanner #'springs-and-rods = ##t
        \once \override DynamicTextSpanner #'minimum-length = #20
        %I tried also the following without success
        %\once \override DynamicTextSpanner #'extra-X-extent = #'(0 . 20)
        %I tried also the following without success
        %\once \override DynamicTextSpanner #'minimum-X-extent = #'(0 . 20)
        c4\< c\f c c
}