Error in data_load.py---TypeError: a bytes-like object is required, not 'str'

[mingze-sheep]

I set prepro=True in hyperparams.py, after running "python3 prepro.py",i begin to run "python3 train.py",then there is a TypeError: a bytes-like object is required, not 'str' ------- File "/home/mingze/tacotron/data_load.py", line 94, in _load_spectrograms mel = "mels/{}".format(fname.replace("wav", "npy")). I can't fix this problem.
PS: when i set prepro=False in hyperparams.py,"python3 train.py" can be run successfully.