Background gradient direction angle = NaN

Adding a background gradient + modifying the direction using the gui tool (the circle with a handle) rather than the arrows makes the direction value display 'NaN', after what manual value editing is broken, possibly because none of the units proposed are relevant (proposed = %, px, em, auto but needs radians or degrees).
Clicking on one of the arrows fixes the ui state, but it is impossible to manually enter an angle value.