kerasのexampleのcnnでValueErrorがでます

Question

kerasのexample、cifar10_cnn.pyで、

Using TensorFlow backend.
x_train shape: (50000, 32, 32, 3)
50000 train samples
10000 test samples
Using real-time data augmentation.
Traceback (most recent call last):
  File "/home/hoge/Desktop/keras-master/examples/cifar10_cnn.py", line 118, in <module>
    workers=4)
  File "/home/hoge/.local/lib/python3.6/site-packages/keras/legacy/interfaces.py", line 91, in wrapper
    return func(*args, **kwargs)
  File "/home/hoge/.local/lib/python3.6/site-packages/keras/engine/training.py", line 1418, in fit_generator
    initial_epoch=initial_epoch)
  File "/home/hoge/.local/lib/python3.6/site-packages/keras/engine/training_generator.py", line 55, in fit_generator
    raise ValueError('`steps_per_epoch=None` is only valid for a'
ValueError: `steps_per_epoch=None` is only valid for a generator based on the `keras.utils.Sequence` class. Please specify `steps_per_epoch` or use the `keras.utils.Sequence` class.

と出ます。

何卒、ご教授宜しくお願いいたします。

#環境
ubuntu 18.04
Python 3.6.6
Keras 2.2.4