Cannot remove a zone from an element using UpdateElement

The UpdateElement and PartialUpdateElement APIs allow changing the image and/or the polygon of the zone of an existing element, but they do not allow removing a zone from an existing element:

>>> cli.request('PartialUpdateElement', id='2e35452b-ecf5-4011-82ad-fc1651fa70e5', body={'image': None, 'polygon': None})
ErrorResponse: {'image': ['This field may not be null.'], 'polygon': ['This field may not be null.']}