Ddoubled parentheses

[Mogens Lemvig Hansen]

Hi,

The NullVoice in the example below generates its own set of parentheses so that 
I get two pairs – see attached.
Same problem with \new NullVoice \with { \remove "Parenthesis_engraver" }{ 
\music }
Same problem in \version "2.19.82"


\version "2.18.2"

music = \relative {
  c' e \parenthesize g c
}

\score {
  \new Staff <<
    \new Voice { \music }
    \new NullVoice { \music }
  >>
}


Suggestions for removing the extra pair – other than by removing the NullVoice 
altogether?

Regards,
Mogens