vtkExtractCellsByType on unstructured grids with one cell type is missing points

Applying the vtkExtractCellsByType filter on an unstructured grid containing a unique type of cell does not contain points.

Python script to reproduce:

import vtk

# Create unstructured grid with one type of cell
cellSource = vtk.vtkCellTypeSource()
cellSource.SetCellType(vtk.VTK_TETRA)
cellSource.Update()

# Extract tetrahedra
extr = vtk.vtkExtractCellsByType()
extr.SetInputConnection(cellSource.GetOutputPort())
extr.AddCellType(vtk.VTK_TETRA)
extr.Update()

# Points are missing
if extr.GetOutput().GetNumberOfPoints() == 0:
    print("Points are missing.")