unused args [...] for include of [...]

edit

I'm trying to pass an argument to a launch file, and I keep getting this error.

The main launch file has something like this:

<?xml version="1.0"?>
<launch>
    <include file="$(find p2os_urdf)/launch/upload_pioneer3at.xml">
        <arg name="foo" value="bar"/>
    </include>
 ...
</launch>

The included file looks like this:

<?xml version="1.0"?>
<launch>
    <param name="testing" value="$(arg foo)"/>
</launch>

Yet I keep getting this error:

unused args [foo] for include of [/home/sciencectn/builds/ros/src/p2os_urdf/launch/upload_pioneer3at.xml]

I assume it means the "foo" argument was never used when it clearly is. What's going on?