Sensor Camera on Gazebo: Could not find parameter robot_description on parameter server

hi all, i'm trying to create a sensor camera on [gazebo](http://www.ros.org/wiki/gazebo), but i have this message on the terminal when i run the launch file: [ERROR] [1323711066.531080823]: Could not find parameter robot_description on parameter server [ERROR] [1323711066.531205567]: Could not generate robot model [ERROR] [1323711066.531238167]: Failed to extract kdl tree from xml robot description [robot_state_publisher-4] process has died [pid 2161, exit code 255]. log files: /home/mauro/.ros/log/0edc0fb2-24e7-11e1-ba97-0016ea548718/robot_state_publisher-4*.log i created a launch file: the file my_camera.lauch is: the file my_camera.urdf .xacro is: true640 480R8G8B8900.0110020.0true20.0finger_tip_cam/imagefinger_tip_cam/camera_infofinger_tip_camera_linktrueGazebo/BlueErratic/YellowErratic/YellowErratic/Yellow