Warning occurs when setting 2d nav goal [closed]

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Hi. I was running a erratic robot simulation and when I try set a goal for the robot using the 2D nav goal in rviz, the following warning occur and I am unable to move my robot.

[ WARN] [1355640188.474944055, 22.497000000]: MessageFilter [target=/map ]:  
Discarding message from [/move_base_node] due to empty frame_id.  This message 
will only print once.

This warning appears in the terminal on which I run the rviz.

Looking up the warning on rxconsole, this is what I get

Node: /rviz_1355641788590219230
Time: 20.566000000
Severity: Warn
Location: /opt/ros/fuerte/stacks/geometry/tf/include 
/tf/message_filter.h:MessageFilter<M>::testMessage:347
Published Topics: /rosout, /move_base_simple/goal, /initialpose, /state

MessageFilter [target=/map ]: Discarding message from [/move_base_node] due to 
empty frame_id.  This message will only print once.

It seems to me this is caused by missing tf frame, but what I don't understand is what message is the warning referring to? From my knowledge, move_base only output cmd_vel and I don't think that's the problem.

Anyone have a solution? I am using Ubuntu 12.04 with ROS fuerte. Thank you.

Update

Hi, after some trial and error, I found out that this is caused by me changing the observation_sources for the costmap in the costmap_common_params.yaml file.