instrumentName in StaffGroup

[Federico Bruni]

I have a StaffGroup containing a Staff and a TabStaff.
I'd like to vertically center the instrument name between the two
staves.
This doesn't work:

\version "2.13.46"

\new StaffGroup <<
  \set StaffGroup.instrumentName = #"Guitar 1 "
  \new Staff { R1 }
  \new TabStaff { R1 }
>>


Why?

It seems that I can use instrumentName only in Staff and TabStaff
contexts.

Thanks,
Federico