\center-align in instrument names

[Graham Percival]

Is this a bug, or am I doing something weird?  \center-align works
fine in Staff.instrument when I have a single instrument, but when I add
a second instrument, the center of \center-align is lined up with the
left side of the other instrument name.  This results in half of the
"clarinetti" running off the page.

{ <<
\new Staff {
  \set Staff.instrument = \markup {
    \center-align { "Clarinetti"
      \line { "in B" \smaller \flat } } }
  c''1
}
\new Staff {
  \set Staff.instrument = "Vibraphone"
  c''1
}
>>
}