When launching simulator Error:Invalid <arg> tag: turtlebot_description

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Hi guys I'm trying to run turtlebot with Gazebo(empty_world) but none of them were working Need help!!!

this is Output when i run this command

       roslaunch turtlebot_gazebo turtlebot_empty_world.launch 

... logging to /home/ss/.ros/log/576cd2c2-8ef2-11e2-9158-00e04c534458/roslaunch-ubuntu-9864.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.

while processing /home/ss/workspace/sandbox/turtlebot_simulator/turtlebot_gazebo/launch/_kobuki.launch:
Invalid <arg> tag: turtlebot_description
ROS path [0]=/opt/ros/groovy/share/ros
ROS path [1]=/home/ss/workspace/sandbox
ROS path [2]=/opt/ros/groovy/share
ROS path [3]=/opt/ros/groovy/stacks. 

Arg xml is <arg default="$(find xacro)/xacro.py '$(find turtlebot_description)/robots/$(arg base)_$(arg stacks)_$(arg 3d_sensor).urdf.xacro'" name="urdf_file"/>