Hi Richard, will test a small case. From my initial investigation the error occurs before the dataset object’s __getitem__ is even called, as the iterator is being setup. I littered the __getitem__ call with print statements and none of them triggered. If I limit the return value of __len__ to the size of one file, it works, it also works for some arbitrary higher value such as 50 million. So am wondering if there some intrinsic limit?