Location_3d_to_region_2d returns None

In my add-on, location_3d_to_region_2d in rare situations returns None. I looked in Blender API:

bpy_extras.view3d_utils.location_3d_to_region_2d(region , rv3d , coord , *** , default=None )

Saw that there is a 4th parameter default which will be returned if coord (3rd parameter) is behind the origin of a perspective view.

Could someone elaborate what it means that a coordinate is behind the origin of a perspective view? Thanks.

Something tells me this is the case.

Ah OK, I had a hard time reproducing any such situation where coord would be behind origin of perspective view, but I managed to cause this situation after making the view focal length really small…