[Bug] Command of evaluating metrics from trained(download) model

Hi! i try to evaluate the metrics of pre-trained model using command `spleeter evaluate -p spleeter:4stems --mus_dir /home/wuxuechao/spleeter/musdb18_stem/ -o eval_output` but get `nan` result. I have download musdb dataset and run the training stage successfully, but i still can't get the results.
By the way, i found that the `_MIXTURE = 'mixture.wav'` command in the 46 lines of `spleeter/commands/evaluate.py/ ` file is a wrong path of musdb dataset, may be there is some misunderstanding in this place?
Thanks a lot if there is any help.