[BUG] negative birth and death values

[caki35]

I have an unweighted and undirected graph obtained via delaunay triangulation. I input 1269x1269 sparse matrix to FlagserPersistence as shown in persistent_homology_graphs.ipynb However, the FlagserPersistence tool with vertex degree filtration gives me a diagram in which both birth and death are negatives.
GF= FlagserPersistence(homology_dimensions=[0, 1], directed=False,filtration = 'vertex_degree')


I expect a sublevel filtration with a monotone increasing vertex degree threshold.