ros urdf tutorials error

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I am following this This tutorial. I installed joint_state_publisher and urdf tutorials. the following command

$ roslaunch urdf_tutorial display.launch model:=urdf/01-myfirst.urdf

is giving following error

Invalid <param> tag: Cannot load command parameter [robot_description]: command [/opt/ros/indigo/share/xacro/xacro.py urdf/01-myfirst.urdf] returned with code [1]. 
Param xml is <param command="$(find xacro)/xacro.py $(arg model)" name="robot_description"/>
The traceback for the exception was written to the log file

So how to solve this error?