What does the transform message mean from rviz laser addin. There are no links by the name hokuyo_frame.

Transform [sender=/gazebo] For frame [hokuyoframe]: Frame [hokuyoframe] does not exist

<plugin name="gazebo_ros_head_hokuyo_controller" filename="libgazebo_ros_laser.so">
        <topicName>/scan</topicName>
        <frameName>hokuyo_frame</frameName>
        <robotNamespace>/rrbot</robotNamespace>
      </plugin>