incorrect voronoi tiling

[blackmad]

I noticed that sometimes my tilings are wrong and managed to get it down to a 4-point case that seems incorrect. One of the polygons it generates only has one edge.

points = [
    (np.float64(20.0), np.float64(20.0)),
    (np.float64(480.0), np.float64(20.0)),
    (np.float64(480.0), np.float64(180.0)),
    (np.float64(20.0), np.float64(180.0))
]

width = 500
height = 200

from foronoi import Polygon, Voronoi

# Create bounding polygon for the window
polygon = Polygon(
    [
        (0, 0),
        (width, 0),
        (width, height),
        (0, height),
    ]
)

# Create Voronoi diagram
voronoi: Voronoi = Voronoi(polygon)
voronoi.create_diagram(points=points)


img = np.zeros((height, width, 3), dtype=np.uint8)

for p in points:
    cv2.circle(img, (int(p[0]), int(p[1])), 1, (255, 255, 255), 3)

for i, site in enumerate(voronoi.sites):
    print(i)
    print(points[i])
    print(site)
    
    # Get cell vertices from the site's borders
    vertices = []
    for edge in site.borders():
        vertex = edge.origin
        vertices.append((int(vertex.x), int(vertex.y)))
    print(vertices)

# I get the same results if I build vertices like this
#    vertices = site.vertices()
#    vertices = [(int(vertex.x), int(vertex.y)) for vertex in vertices]
#    print(vertices)
    
    if len(vertices) > 2:  # Need at least 3 vertices for a polygon
        next_vertices = [*vertices[1:], vertices[0]]
        for v1, v2 in zip(vertices, next_vertices):
            cv2.line(img, v1, v2, (255, 255, 255), 1)

imshow(img)

outputs:

0
(np.float64(20.0), np.float64(20.0))
P2
[(0, 100)]
1
(np.float64(480.0), np.float64(20.0))
P3
[(250, 100), (500, 100), (500, 0), (0, 0), (0, 100)]
2
(np.float64(480.0), np.float64(180.0))
P1
[(250, 100), (250, 200), (500, 200), (500, 100)]
3
(np.float64(20.0), np.float64(180.0))
P0
[(250, 200), (250, 100), (0, 100), (0, 200)]