vtkExtractUnstructuredGrid with polyhedron bug

import vtk

# Prepare the unstructured grid --------------------------------------------------------------------
grid = vtk.vtkUnstructuredGrid()

points = vtk.vtkPoints()
points.SetNumberOfPoints(8)
points.SetPoint(0, 0, 0, 0)
points.SetPoint(1, 1, 0, 0)
points.SetPoint(2, 1, 1, 0)
points.SetPoint(3, 0, 1, 0)
points.SetPoint(4, 0, 0, 1)
points.SetPoint(5, 1, 0, 1)
points.SetPoint(6, 1, 1, 1)
points.SetPoint(7, 0, 1, 1)
grid.SetPoints(points)

grid.Allocate(1)

def AddPolyhedronTo(grid):
    polyhedron = [
        [0, 3, 2, 1],
        [0, 4, 7, 3],
        [4, 5, 6, 7],
        [5, 1, 2, 6],
        [0, 1, 5, 4],
        [2, 3, 7, 6],
    ]
    polygon_faces = vtk.vtkCellArray()
    for face in polyhedron:
        q = vtk.vtkPolygon()
        q.GetPointIds().SetNumberOfIds(len(face))
        for index, edge in enumerate(face):
            q.GetPointIds().SetId(index, edge)
        polygon_faces.InsertNextCell(q)

    face_stream = vtk.vtkIdList()
    face_stream.InsertNextId(polygon_faces.GetNumberOfCells())
    vertex_list = vtk.vtkIdList()

    polygon_faces.InitTraversal()
    while polygon_faces.GetNextCell(vertex_list) == 1:
        face_stream.InsertNextId(vertex_list.GetNumberOfIds())
        for j in xrange(vertex_list.GetNumberOfIds()):
            face_stream.InsertNextId(vertex_list.GetId(j))
    grid.InsertNextCell(vtk.VTK_POLYHEDRON, face_stream)

def AddHexahedronTo(grid):
    aHexahedron = vtk.vtkHexahedron()
    aHexahedron.GetPointIds().SetId(0, 0)
    aHexahedron.GetPointIds().SetId(1, 1)
    aHexahedron.GetPointIds().SetId(2, 2)
    aHexahedron.GetPointIds().SetId(3, 3)
    aHexahedron.GetPointIds().SetId(4, 4)
    aHexahedron.GetPointIds().SetId(5, 5)
    aHexahedron.GetPointIds().SetId(6, 6)
    aHexahedron.GetPointIds().SetId(7, 7)
    grid.InsertNextCell(aHexahedron.GetCellType(), aHexahedron.GetPointIds())

# Choose one of them, comment the other ------------------------------------------------------------
AddPolyhedronTo(grid)
# AddHexahedronTo(grid)

# Test the algorithm -------------------------------------------------------------------------------
unstructured_grid_extractor = vtk.vtkExtractUnstructuredGrid()
unstructured_grid_extractor.SetInputData(grid)
unstructured_grid_extractor.SetCellMinimum(0)
unstructured_grid_extractor.SetCellMaximum(0)
unstructured_grid_extractor.PointClippingOff()
unstructured_grid_extractor.CellClippingOn()
unstructured_grid_extractor.ExtentClippingOff()
unstructured_grid_extractor.MergingOff()
unstructured_grid_extractor.Update()
output = unstructured_grid_extractor.GetOutput()

assert grid.GetCell(0).GetNumberOfPoints() == output.GetCell(0).GetNumberOfPoints()