Gazebo Crashes when modifying joint velocities [closed]

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Hi All

Just trying to test a plugin to a sdf model of a 7DOF manipulator. In order to test, my i've included the following line in the plugin's OnUpdate() method:

joint1->SetVelocity(0, 0.001);

There are a few other lines in that method but they dont crash the system. joint1 is a physics::JointPtr and SetVelocity is a method of a joint. I am using rosmake, and it seems to compile fine. However when I go to start GAZEBO I get the following message:

gzserver: /usr/include/boost/smart_ptr/shared_ptr.hpp:418: T* boost::shared_ptr<t>::operator->() const [with T = gazebo::physics::Joint]: Assertion `px != 0' failed. Aborted (core dumped)

Any suggestions