Octava clef "8"

[Aurèle]

Hello,
On lilypond 2.2.2 with Cygwin :
This file produces bad output: the "8" stay in the first line (between 
the two Staff of the PianoStaff) therefore its staff is removed by  
\RemoveEmptyStaffContext.

\version "2.2.2"
\score{\notes
       <<
           \new Staff \notes{\clef "G_8" R1 a1 }
           \new PianoStaff <<
               \new Staff \notes{ a1 \break a1 }
               \new Staff \notes{ a1 a1 }
           >>
       >>
       \paper{
           \context{ \RemoveEmptyStaffContext }
       }
   }