ROS Init Exception

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Hi all!

I wrote two nodes in python and get the following exception when I start the nodes.

Can somebody tell me whats the problem?

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/opt/ros/electric/stacks/geometry/tf/src/tf/listener.py", line 238, in     run
    rospy.spin()
  File "/opt/ros/electric/stacks/ros_comm/clients/rospy/src/rospy/client.py", line 98, in spin
    raise rospy.exceptions.ROSInitException("client code must call    rospy.init_node() first")
ROSInitException: client code must call rospy.init_node() first

Thanks for help!