Gazebo crashes when sensors are added to ShadowHand

edit

After adding the following section to model/hand_urdf/thumb/thdistal.urdf.xacro in the sr_hand package Gazebo crashes during startup.

<gazebo reference="thdistal">
  <sensor:contact name="thdistal_contact_sensor">
    <geom>distal_collision_geom</geom>    
    <updateRate>100.0</updateRate>    
    <controller:gazebo_ros_bumper name="thdistal_gazebo_ros_bumper_controller" plugin="libgazebo_ros_bumper.so">
      <alwaysOn>true</alwaysOn>    
      <updateRate>100.0</updateRate> 
      <frameName>thdistal</frameName>  
      <bumperTopicName>thdistal_bumper</bumperTopicName>
      <interface:bumper name="thdistal_gazebo_ros_bumper_iface" />
    </controller:gazebo_ros_bumper>    
  </sensor:contact>
  <material>Gazebo/Grey</material>
</gazebo>

If I comment out the <geom> and the first <updaterate> tags Gazebo starts, but no contact messages are published.

All log files indicate that everything should be fine.

Any idea what I'm doing wrong?