get zones by point doubt

I execute the request resource/get_zones_by_point with radius param = 0 and I get the response

[resourceid] => Array
                (
                    [0] => 126
                )

Acording to the oficial documentation the index  of array is the geofence id but the id 0 "zero" not look like a geofence id

But when I execute this same request with radius param = 100 I get this response

            [resourceid] => Array
                (
                    [126] => 0
                )

Really i don't understand this behavior.