gazebo_ros_bumper_controller not updating bumper states

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I have been implementing this code. I am trying to echo the Contact messages but states does not echo any value even if it undergoes collision.Can someone please tell me where I am going wrong

<gazebo reference="base">
<sensor name="main_bumper" type="contact">
<geom>base_geom</geom>
    <selfCollide>true</selfCollide>
    <alwaysOn>true</alwaysOn>
    <updateRate>100.0</updateRate>
    <material>Gazebo/Red</material>
    <contact>
       <collision name="bump_sensor_collision_collision"></collision>
    </contact>
    <plugin name="gazebo_ros_bumper_controller" filename="libgazebo_ros_bumper.so">   
        <bumperTopicName>bumper_vals</bumperTopicName>
        <frameName>world</frameName>
    </plugin>
</sensor>
</gazebo>

</robot>

This is the output that I am getting :

header: 
  seq: 89604
  stamp: 
    secs: 584
    nsecs: 451000000
  frame_id: world
states: []