I was having this issue. Turns out its because there was an error in the dataset object (for me it was in the __getitem__ function). I guess the DataLoader in multiprocessing mode doesn’t know how to cleanly provide you with the internal error message. If you have the same problem, try running with num_workers = 0 (single-threaded) and it should tell you what the error is. Once you’ve fixed the error, it should work with num_workers > 0.