Black Tiles in Mosaic

I have followed you code in the example: ```python from phomo import Master, Pool, Mosaic pool = Pool.from_dir("quadratische_bilder/", crop_ratio=1, tile_size=(200, 200)) master = Master.from_file("master1.jpeg", img_size=(5712*3, 4284*3)) mosaic = Mosaic(master, pool) mosaic.grid.plot() d_matrix = mosaic.d_matrix(workers=4) mosiac_img = mosaic.build(d_matrix) mosiac_img ``` but I got the following result:  You see the black tiles, how can I fix them?