The following .world and model.sdf fails to load the second model and Gazebo does not print an error.

Is this invalid SDF? I think so as the included model:// is missing a element, but ideally Gazebo wouldn't fail silently. I tried file:// URI too.

// house.world
<sdf version="1.6">
  <world name="my_house">
    <include>
      <uri>model://sun</uri>
    </include>
    <include>
      <uri>model://ground_plane</uri>
    </include>
    <include>
      <uri>model://my_room</uri>
    </include>
  </world>
</sdf>

// my_room/model.sdf 
<sdf version="1.6">
  <include>
    <uri>model://bookshelf</uri>
    <pose>0 0 1 0 0 0.0</pose>
  </include>
  <include>
    <uri>model://cafe_table</uri>
    <pose>0 4 1 0 0 0.0</pose>
  </include>
</sdf>