Century Schoolbook not italic

[Noeck]

Hi,

why does this example code not work? I.e. why does it not print the clef
modifier 8 in the italic version of the Century Schoolbook font?

\version "2.18.0"
\score {
  { \clef "treble_8" a1^"8 is not italic" }
  \layout {
    \override Staff.ClefModifier.font-name = #"Century Schoolbook L"
    \override Staff.ClefModifier.font-shape = #'italic
  }
}

This works, but for other fonts the font-shape can be changed without
changing the name.

\score {
  { \clef "treble_8" a1^"8 is italic" }
  \layout {
    \override Staff.ClefModifier.font-name = #"Century Schoolbook L italic"
  }
}

TIA,
Joram