[ros-users] tf spits a lookup exception

Hello,
I'm not sure whether this is better asked at answers.ros.org, but since it
looks like a bug I'm sending it here

I'm using the tf package and specifically the tf.TransformerROS class in
Python.
If I try calling its methods transformPose or transformPointCloud it returns
a LookupException

    tf.LookupException: Frame id /map does not exist! When trying to
transform between /base_link and /map.

However if I try running the tf_monitor right after that I see that tf has
knowledge of the frame

    ~>rosrun tf tf_monitor base_link map
    Waiting for transform chain to become available between /base_link and
/map

    RESULTS: for /base_link to /map
    Chain is: /base_link -> /base_footprint -> /odom_combined -> /map
    Net delay     avg = 0.00684423: max = 0.054

    Frames:
    Frame: /base_footprint published by /robot_pose_ekf Average Delay:
0.0185 Max Delay: 0.026
    Frame: /base_link published by /robot_state_publisher Average Delay:
0.0075 Max Delay: 0.008
    Frame: /odom_combined published by /slam_gmapping Average Delay: 0 Max
Delay: 0

    All Broadcasters:
    Node: /robot_pose_ekf 33.5196 Hz, Average Delay: 0.0185 Max Delay:
0.026
    Node: /robot_state_publisher 55.5556 Hz, Average Delay: 0.0075 Max
Delay: 0.008
    Node: /slam_gmapping 25 Hz, Average Delay: 0 Max Delay: 0

I'm running gazebo simulation and have ROBOT=sim set.

Best,
Dimitar