xacro.py generates empty urdf file

I am using following command to convert *.urdf.xacro to *.urdf in ROS indigo,ubuntu 14.04: rosrun xacro xacro.py a.urdf.xacro > a.urdf also rosrun xacro xacro.py -o a.urdf a.urdf.xacro content of a.urdf.xacro(just for test): and the content of generated urdf: I have faced this problem in two different machines. any idea?