raise ImageFileError(f'Cannot work out file type of "{filename}"') when working with nnunet

Question

I'm working with nnunet network with a custom dataset. Everything works fine, I've created the new Task 88, I've launched the !nnUNet_plan_and_preprocess -t 88 --verify_dataset_integrity command and all the training cases and after the labels are checked, the problem is that; after this, it shows this error:

Verifying test set
Traceback (most recent call last):
  File "/home/viberti/miniconda3/bin/nnUNet_plan_and_preprocess", line 8, in <module>
    sys.exit(main())
  File "/home/viberti/miniconda3/lib/python3.9/site-packages/nnunet/experiment_planning/nnUNet_plan_and_preprocess.py", line 105, in main
    verify_dataset_integrity(join(nnUNet_raw_data, task_name))
  File "/home/viberti/miniconda3/lib/python3.9/site-packages/nnunet/preprocessing/sanity_checks.py", line 223, in verify_dataset_integrity
    all_same, unique_orientations = verify_all_same_orientation(join(folder, "imagesTr"))
  File "/home/viberti/miniconda3/lib/python3.9/site-packages/nnunet/preprocessing/sanity_checks.py", line 34, in verify_all_same_orientation
    img = nib.load(n)
  File "/home/viberti/miniconda3/lib/python3.9/site-packages/nibabel/loadsave.py", line 55, in load
    raise ImageFileError(f'Cannot work out file type of "{filename}"')
nibabel.filebasedimages.ImageFileError: Cannot work out file type of "/home/viberti/nnUNet_raw_data_base/nnUNet_raw_data/Task088_BraTS2020/imagesTr/BRATS_001_0000.nii.gz"