Rigidbody.velocity giving weird result

The following method, when called in fixed update, prints vector3.zero and zero to the console every second call. Otherwise it prints the correct( or at least expected ) values to the console.

Any clues why?

 private void getVelocity()
     {
         // get the present velocity
         currentVelocity = rigidbody.velocity;
         // get the speed
         currentSpeed = Vector3.Magnitude(currentVelocity);
         // last get old position 
         // oldPosition = transform.position; 
         Debug.Log("vel = " + currentVelocity);
         Debug.Log("spe = " + currentSpeed);
     }

PLEASE NOTE: I require the velocity vector for dampening calculation which has not been created yet. I will accept alternatives that give the same vector 3 information.

Also it would be helpful to know if this is reproducible on other systems so I can completely rule out other code interfering.

Thanks.