contact sensor has no member named contacts

I was following [this](http://gazebosim.org/tutorials?tut=contact_sensor) tutorial , but as i try to build the code i get theis error msg: /usr/include/boost/asio/detail/impl/socket_ops.ipp:266:5: note: ‘boost::asio::detail::socket_ops::bind’ int bind(socket_type s, const socket_addr_type* addr, ^ /home/rob/Documents/gazebo_contact_tutorial/ContactPlugin.cc: In member function ‘virtual void gazebo::ContactPlugin::OnUpdate()’: /home/rob/Documents/gazebo_contact_tutorial/ContactPlugin.cc:43:34: error: ‘class gazebo::sensors::ContactSensor’ has no member named ‘Contacts’ contacts = this->parentSensor->Contacts(); How can i remove this error? I am using ros indigo with gazebo 2.2.