Problem with NXT tutorial

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Hi, I'm trying to follow this tutorial but when I launch the program I get this warning:

[INFO] [WallTime: 1335361633.048001] Creating ultrasonic with name 
  ultrasonic_sensor on PORT_2
[WARN] [WallTime: 1335361633.284565] caught an exception nxt.error.DirProtError

My yaml file has this characteristics:

- type: ultrasonic
  frame_id: r_ultrasonic_link
  name: ultrasonic_sensor
  port: PORT_2
  spread_angle: 0.2
  min_range: 0.01
  max_range: 2.5
  desired_frequency: 10.0

Supposedly, these lines creates an ultrasonic sensor with a spread angle of 0.2m, a min range of 0.01m, and a max range of 2.5m that is connected to PORT_2.

I only has this problem with the ultrasonic sensor. Touch and color (intensity) sensors work fine. I have the correct firmware (v1.28).

EDIT: I've listed the topics:

$ rostopic list
/intensity_sensor
/joint_command
/joint_state
/joint_states
/rosout
/rosout_agg
/touch_sensor
/ultrasonic_sensor

But when I try to see the topic I get this warning.

$ rostopic echo /ultrasonic_sensor
WARNING: topic [/ul/ultrasonic_sensor] does not appear to be published yet

Thanks in advantage for your help.

Cheers,

Raúl Pérula