forced break was overridden by some other event

[Federico Bruni]

Hi folks

What's wrong with this input?
I cannot force line break and I get the warning "forced break was 
overridden by some other event":

\version "2.19.48"

upper = \relative {
 e'8 cis( b) gis~ gis r b4
   % maybe it's the grace synchronization bug?
   \hideNotes \grace b8 \break |
 r1
}

lower = \relative {
 e,2 e'8 r g4\4\glissando
   % start glissando which will continue on a previous bar
   \hideNotes \grace gis8\4 \unHideNotes \break |
 e,2 e'^\fermata |
}

\score {
 \new StaffGroup <<
   \new Staff { \clef "treble_8" << \voiceOne \upper \\ \voiceTwo 
\lower >> }
   \new TabStaff \with {
     stringTunings = #guitar-tuning
     \clef moderntab
   } <<
     \new TabVoice { \voiceOne \upper }
     \new TabVoice { \voiceTwo \lower }
   >>
 >>
}