I am needing to get the current camera angle so am using the current direction as a Cartesian3 and checking its angleBetween back to (0, 0, 0).

oldBearing = Cesium.Cartesian3.angleBetween(viewer.scene.camera.direction, Cesium.Cartesian3.ZERO);

The formula works fine for any Cartesian3 (right) that is not (0, 0, 0) but the result is NaN using ZERO.

Please advise with thanks.