Unable to find uri[model://model:://sun]

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I've just started learning ROS and GAZEBO. I'm using Ubuntu 16.04.4, ROS Lunar and Gazebo 7.

The following world:

<?xml version="1.0"?>
<sdf version="1.4">
    <world name="myworld">
        <include>
            <uri>model://model:://sun</uri>
        </include>

        <include>
            <uri>model://model:://ground_plane</uri>
        </include>
    </world>
</sdf>

Throws these errors:

Error [parser.cc:581] Unable to find uri[model://model:://sun]
Error [parser.cc:581] Unable to find uri[model://model:://ground_plane]

Both models are in ~/.gazebo/models/.

I have also tried this world file:

<sdf version="1.4">
    <world name="myworld">
        <include>
            <uri>model:://sun</uri>
        </include>

        <include>
            <uri>model:://ground_plane</uri>
        </include>
    </world>
</sdf>

Getting the same error.

Do I need to do anything else? Like adding the GAZEBO_MODEL_PATH env var into the world file.