<gripper> tag (gazebo::physics::Gripper) doesn't create the fixed joint between the gripper and the object within it

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I have been trying to use the <gripper> tag in my urdf robot like this:

<gazebo>
    <gripper name="grasping">
        <grasp_check>
            <attach_steps>20</attach_steps>
            <detach_steps>40</detach_steps>
            <min_contact_count>1</min_contact_count>
        </grasp_check>
        <gripper_link>r_finger_link</gripper_link>
        <gripper_link>l_finger_link</gripper_link>
        <palm_link>gripper_base</palm_link>
    </gripper>
</gazebo>

I have checked the corresponding code and it's loaded properly. The problem is that the Gripper::OnContact function is never called, therefore the fixed joint between the gripper and the object within the gripper will never be created.

The contacts do exist as can be seen in the figure. Additionally when I use a ContactSensor like the libgazebo_ros_bumper.so plugin I also detect such contacts.

Has somebody run with the same problem?

I'm working with gazebo-1.9.1, ros-groovy and gazebo_ros_pkg