multi turtlebot in gazebo and rviz problem [closed]

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i,m usinf fuerte and ubuntu 12.04. i have seen this post http://answers.ros.org/question/41433/multiple-robots-simulation-and-navigation/ to use two turtlebots in rviz. i have done it correctly , the only problem i,m having is the warning message below when i run the launch file.

WARNING: Waiting on transform from /robot1_tf/base_link to /robot1_tf/map to become available before running costmap, tf error:

WARNING: Waiting on transform from /robot2_tf/base_link to /robot2_tf/map to become available before running costmap, tf error

hope somebody will help.