Preprocessing Error occurring for using custom dataset

[Shehan-Irteza-Pranto]

Traceback (most recent call last):
File "preprocess.py", line 110, in
main()
File "preprocess.py", line 106, in main
run_preprocess(args, modified_hp)
File "preprocess.py", line 83, in run_preprocess
preprocess(args, input_folders, output_folder, hparams)
File "preprocess.py", line 17, in preprocess
metadata = preprocessor.build_from_path(hparams, input_folders, mel_dir, linear_dir, wav_dir, args.n_jobs, tqdm=tqdm)
File "/media/aims/New_Volume/PRANTO/tts/tecetron_v2/Tacotron-2/datasets/preprocessor.py", line 32, in build_from_path
for input_dir in input_dirs:
TypeError: 'NoneType' object is not iterable