Problem with OnPlatform (Labs)

Im trying to set the WidthRequest differently on each platform, eg:

  <toolkit:ImageButton.WidthRequest>
    <OnPlatform x:TypeArguments="x:Int32" iOS="30" Android="30" WinPhone="75" />
  </toolkit:ImageButton.WidthRequest>

however it doesn't seem to work. I keep getting 0 for the WidthRequest.

what should I put in x:TypeArguments?