getPosition() return NaN

Joint joint = physicsSpace.createJoint();
joint.attach( node1, node2 );
joint.setAnchor( anchor );
leftShoulderAxis = joint.createRotationalAxis();
leftShoulderAxis.getPosition()  <


NaN



It always returns NaN for me. I want it to implement a servo: get relative angle with attachment, subtract from desired angle , setDesiredVelocity(-difference*20).

Anyone else has this problem?

Thanks!