ros-by-example vol2. roslaunch in CH4 xacro error

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ros-by-example vol2.

CH4 CREATING A URDF MODEL FOR YOUR ROBOT

The command line is shown below:

$ roslaunch rbx2_description box_robot_base_only.launch

The error message is shown below:

 root@ShawnVM:~/catkin_ws/src/rbx2/rbx2_dynamixels# roslaunch rbx2_description box_robot_base_only.launch 
... logging to /root/.ros/log/0d82b658-a196-11e6-9533-000c290a3fdb/roslaunch-ShawnVM-48840.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

Invalid <arg> tag: xacro
ROS path [0]=/opt/ros/indigo/share/ros
ROS path [1]=/root/catkin_ws/src
ROS path [2]=/opt/ros/indigo/share
ROS path [3]=/opt/ros/indigo/stacks. 

Arg xml is <arg default="$(find xacro)/xacro.py '$(find rbx2_description)/urdf/box_robot/box_robot_base_only.xacro'" name="urdf_file"/>
The traceback for the exception was written to the log file