tfListener can't find frames

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Hello everyone,

in reference to a previous question, I wrote this simple urdf file:

    <robot name="sonar_platform">

    <!-- base link -->
    <link name="base_link" />

    <!-- sonar-ir fixed links -->
    <link name="sonar0_link" />
    <link name="sonar1_link" /> 

    <joint name="sonar0_joint" type="fixed">
        <parent link="base_link" />
        <child link="sonar0_link" />
        <origin xyz="1 1 0" rpy="0 0 0"/>
    </joint>

    <joint name="sonar1_joint" type="fixed">
        <parent link="base_link" />
        <child link="sonar1_link" />
        <origin xyz="1 -1 0" rpy="0 0 1.57"/>
    </joint>
</robot>

Then I use robot_state_publisher to publish the trasforms to tf using a very simple launch file:

    <launch>
    <param name="robot_description" command="cat $(find sonar_array)/platform_urdf.xml" />
        <node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher" />
</launch>

Finally, I implemented a tf listener based on the tutorial

#include <ros/ros.h>
#include <tf/transform_listener.h>

int main(int argc, char** argv){
  ros::init(argc, argv, "sonar_array");

  ros::NodeHandle node;

  tf::TransformListener listener;

  ros::Rate rate(10.0);
  while (node.ok()){
    tf::StampedTransform transform;
    try{
        listener.lookupTransform("/base_link", "/sonar0_link",  
                               ros::Time(0), transform);
    }
    catch (tf::TransformException ex){
      ROS_ERROR("%s",ex.what());
    }
}

but when I try to launch it I get the following error:

    [ERROR] [1312225092.766820278]: Frame id /base_link does not exist! When trying to transform between /sonar0_link and /base_link.
terminate called after throwing an instance of 'tf::LookupException'
  what():  Frame id /sonar0_link does not exist! When trying to transform between /sonar0_link and /base_link.
Aborted

While, if I subscribe the /tf topic i can correctly see the transforms I defined:

    transforms: 
  - 
    header: 
      seq: 0
      stamp: 
        secs: 1312225269
        nsecs: 108000004
      frame_id: /base_link
    child_frame_id: /sonar0_link
    transform: 
      translation: 
        x: 1.0
        y: 1.0
        z: 0.0
      rotation: 
        x: 0.0
        y: 0.0
        z: 0.0
        w: 1.0
  - 
    header: 
      seq: 0
      stamp: 
        secs: 1312225269
        nsecs: 108000004
      frame_id: /base_link
    child_frame_id: /sonar1_link
    transform: 
      translation: 
        x: 1.0
        y: -1.0
        z: 0.0
      rotation: 
        x: 0.0
        y: 0.0
        z: 0.706825181105
        w: 0.707388269167
---

Any ideas on what's going on?