Cannot convert from numpy type.Unknown data type is returned!

def load_batch(files):
    for file in x:
        _ = pkl.load(file)
    ...
    return ouput1, output2, outpu3

train_files = ['1.pkl', '2.pkl', ...]   # 训练文件的list
train_data = ds.NumpySlicesDataset(train_files, column_names=['files'])
train_data = train_data.batch(batch_size)
train_data = train_data.map(lambda x:load_batch(x),
                            input_columns=['files'],
                            output_columns=['output_1', 'output2', 'output3'])