Markers does not appear when Infowindow with partial added

Question

I am using Gmaps4rails gem. I could show markers without Infowindow, but after adding Infowindow in the controller, any markers do not appear. As the following, the Infowindow is rendered with partial.

What should I change the code? Thank you in advance.

clinics_controller.rb

@hash = Gmaps4rails.build_markers(@clinics) do |clinic, marker|
  marker.lat clinic.latitude
  marker.lng clinic.longitude
  marker.json({:id => clinic.id})
  marker.picture ({
    "url" => "/images/red-dot.png",
    "width" => 32,
    "height" => 32
  })
  marker.infowindow render_to_string(partial: "map_infowindow", locals: {clinic: clinic})
end

_map_infowindow.html.erb

<h3><%= clinic.name %></h3>

Javascript

<script type="text/javascript" charset="utf-8">
  handler = Gmaps.build('Google');

  handler.buildMap({
    provider: {
      zoom: 15,
      center: {lat: 35.765, lng: 139.62}
    },
    internal: {id: 'map'}},
    function(){
      markers = handler.addMarkers(<%= raw @hash.to_json %>);
    }
  );
</script>