rospy: how to set a transform to TransformerROS ?

edit

After initializing a node, a declare a TransformerROS.

transformer = tf.TransformerROS(True,rospy.Duration(10.0))

but if I want to perform a transform between two frames (in my case, I use turtlebot and I want to transform between odom and base_link), this does not work :

transformed_point = transformer.transformPoint("odom",point)

with error:

[...]
File "/opt/ros/indigo/lib/python2.7/dist-packages/tf/listener.py", line 71, in asMatrix
translation,rotation = self.lookupTransform(target_frame, hdr.frame_id, hdr.stamp)
[...]
LookupException: "odom" passed to lookupTransform argument target_frame does not exist

note: I have the bring up nodes of turtlebot running and rostopic informs me odom is broadcasted at 50Hz. Also odom frame shows up in rviz.

Also, I notice :

 transformer.getFrameStrings()

prints []

What am I missing ?

Example from here: http://docs.ros.org/jade/api/tf/html/... seems to imply I should set the transform to the transformer first, but according to the error I get, the transformer seems to be searching for the transform itself (" ... self.lookupTransform(target_frame, hdr.frame_id, hdr.stamp) ...")

edit : I tried with an instance of TransformListener rather than TransformROS, for same result