Pickling DataBlock throws an error

Instantiation of DataBlock takes around 10 minutes. For this reason, I want to pickle it for later reuse:

path = untar_data(URLs.IMDB)
db = DataBlock(
       blocks=(TextBlock.from_folder(path), CategoryBlock),
       get_y = parent_label,
       get_items=partial(get_text_files, folders=[‘train’, ‘test’]),

with open('db_full.pickle', 'wb') as handle:
    pickle.dump(db, handle)

pickle.dump throws the following error.
AttributeError: Can't pickle local object 'GrandparentSplitter.<locals>._inner'
Any idea why I can’t save an instance?