stem extend doe not work

[Stefan Thomas]

Dear Lilypond-users,
I tried the thing with  stems extended over two piano staves.
But the below quoted exemple did'nt work and I do not know the
reason. Could You give me an advice?
Thanks,
and merry Christmas! Stefan

Here the short example:
stemExtend = {
       \once \override Stem #'length = #20
       \once \override Stem #'cross-staff = ##t
     }
     noFlag = \once \override Stem #'flag-style = #'no-flag

     \new PianoStaff <<
       \new Staff \relative c' { c8.  c16~  c16  c8 des16~ des2

       }
       \new Staff \relative c{
         \clef bass \stemUp  \stemExtend fis8. fis16~ \stemExtend
fis16 fis8 f16~ f2

 }
     >>