Trill spanner minimum length problem

[Menu Jacques]

Hello folks,

In a Poulenc score, I have the trill displayed in the attached PNG, but narrower due to LP’s setting of the line breaks, which makes the prall quite short:

I tried to set a minimum length with the following, but to no avail: the result is the same whatever the value I supply:

\version "2.19.28"

\relative c'' {
  \override TrillSpanner.minimum-length = #38 % <<<—-— HERE

  c4 -\markup{ \override #`(direction . ,DOWN) \fermata} ^\markup{\natural \raise #-0.5 \italic "serré"}
  \startTrillSpan c16 \stopTrillSpan r16 \fermata bes8 -.

  \bar "||"
}

Thanks for your help!

JM