programming error : no heads for arpeggio found ?

[philippe hezaine]

version 2.8.5
      Hi list,
I don't understand why i've this message in this little polyphonic extract.
Lilypond compile but programming error is always annoying.
And i haven't found anything in the archives  with this thread.
        
Calculating line breaks... [2]
programming error: no heads for arpeggio found?
continuing, cross fingers
programming error: no heads for arpeggio found?
continuing, cross fingers
Calculating page breaks...  \version "2.8.5"

 {
   \set PianoStaff.connectArpeggios = ##t
   <<{e''16\arpeggio^\( dis''\) cis''^.^\( b'^.\) b'8\noBeam b'16^.^\( 
cis''^.\)}
         \\{a'4\arpeggio b'16_\( a'\) gis'_._\( fis'_._\)}>>
}       

It's an extract from a complete ly. file
I also tried with \set Voice.connectArpeggios in the \score block. I 
obtain the same message.
Is anyone knows the reason why ? What have i done wrong ?