I went ahead and dug in to the Client object a bit more, and was able to investigate the cause of this problem. Apparently, if you are going to call histories.show_history() to retrieve history-specific data, you can't use the 'dataset_id' field as the dataset_id in histories.delete_dataset(). You actually want to use the 'id' key. This was clear after I printed our the contents of the json blob response that was given with my history_id and dataset_id parameters. This may be worth notating in the documentation. :-)