marker.shadow json builder

[dudo]

@hash = Gmaps4rails.build_markers(@events) do |event, marker|
    marker.lat event.lat
    marker.lng event.lng
    marker.title "#{pluralize(event.attendees.size, 'person')} going"
    marker.infowindow render_to_string(partial: '/events/infobox', 
                                       locals: {object: event})
    marker.picture({
      anchor: [10,34],
      url: '/assets/purple_marker.png',
      width: 20,
      height: 34 })
    marker.shadow({
      anchor: [2,22],
      url: '/assets/sprite_shadow.png',
      width: 29,
      height: 22 })
  end

undefined method 'shadow' for #<Gmaps4rails::MarkersBuilder::MarkerBuilder:0x455f670>

If I remove marker.shadow, the pictures build fine, but no shadow, obviously.