Y-offset not working for slurs

[Nao Nao]

Hi,

Does anybody know how to increase the slur vertical space without using control points?

Tried this below with no success.

\version "2.18.2"
\relative c' {
   \override PhrasingSlur.direction = #UP
   \override PhrasingSlur.Y-offset = #10 %{removing this line is not making any difference,
   I want to make the slur higher than normal %}
   f8\( d f d g d f d\)
}

Cheers,

Thiago