Strange Behavior when setting link states

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Hi,

I have currently made a simple urdf which contains 2 links and a joint. The idea is to have one of the links rotate around the z axis. Everything loads correctly in the gazebo gui, however, when I attempt to set the orientation (via rostopic pub /gazebo/set_link_state) my model orients itself correctly but flies up in the air.

Would anyone know why this is happening and how I can fix it? (I can provide my files if necessary)

Thanks!