OnPlatform in System:Double, possible ?

App.xaml:

  <System:Double x:Key="FontSizeSmall">
    <OnPlatform x:TypeArguments="System:Double" Android="10" iOS="9" />
  </System:Double>

Page1.xaml:

   <Label Text="MyText" FontSize="{StaticResource FontSizeSmall}"  />

This does not work.

If i use this it does work:

  <System:Double x:Key="FontSizeSmall">10</System:Double>

What am i doing wrong ?