ERROR while loading world with DiffDrivePlugin as model plugin

Hi there,

I'm trying to control a Pioneer2dx with *DiffDrivePlugin*. Whenever I start the server, an error occurs. I got the plugin from [here](https://bitbucket.org/osrf/gazebo/src/b60a5186d0008a915ccdc632f17df5d09ca33c24/plugins/DiffDrivePlugin.cc?at=default&fileviewer=file-view-default). No errors are coming while complilng.
Error:
**Error [Element.cc:671] Missing element description for [left_joint]
gzserver: /usr/include/boost/smart_ptr/shared_ptr.hpp:648: typename boost::detail::sp_member_access::type boost::shared_ptr::operator->() const [with T = sdf::Element; typename boost::detail::sp_member_access::type = sdf::Element*]: Assertion `px != 0' failed.
Aborted**

What is this error and why this is coming? Can anyone provide me a solution?

Thanks in advance!