Cannot read IWSLT dataset

Hi, 

The following code does not execute: 
"""
    train_data, valid_data, test_data = datasets.IWSLT.splits(
        exts=('.de', '.en'), fields=(SRC, TRG), 
        filter_pred=lambda x: len(vars(x)['src']) <= MAX_LEN and 
            len(vars(x)['trg']) <= MAX_LEN
"""
What should I replace this code with? Thanks for a great code base. 