TF: request latest transform -> interpolation into the future

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As stated in the tutorial the following code

listener.waitForTransform( 'X', 'Y', rospy.Time(), rospy.Duration(5))
(a,b) = listener.lookupTransform( 'X', 'Y', rospy.Time())

should wait until any transformation between X and Y available is and then return it. But instead I get ExtrapolationException: Lookup would require extrapolation into the future.

What do I have to do to get just the latest available transformation?

The problem occurs when I publish joint values and then try to get a transformation published by the robot_state_publisher. Everythin is on the same pc.