They were previously json.dumped by us to contentstore in ```save_subs_to_store``` in ```transcripts_utils.py: 66```. I assume if they were dumped successfully, they will be loaded successfully. If ```json.dumps``` will raise exception in ```save_subs_to_store```, then content will not be saved and NotFoundError should be raised in line 271. I've added test for that, please look at https://github.com/edx/edx-platform/commit/57aa030be5e07faab8691dcb79ce986a47adf792