Not able to save model for the quickstart example

Here is the quickstart example: https://www.tensorflow.org/recommenders/examples/quickstart

It works fine, but I am not able to save the model by model.save('my_model') at the end. It throws exception:

WARNING:tensorflow:Skipping full serialization of Keras layer <__main__.MovieLensModel object at 0x7f727eec7690>, because it is not built.
WARNING:tensorflow:Skipping full serialization of Keras layer <__main__.MovieLensModel object at 0x7f727eec7690>, because it is not built.
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-8-5a8adf13c269> in <module>()
----> 1 model.save("my_model")

3 frames
/usr/local/lib/python3.7/dist-packages/tensorflow/python/keras/saving/saving_utils.py in raise_model_input_error(model)
     96       'set. Usually, input shapes are automatically determined from calling'
     97       ' `.fit()` or `.predict()`. To manually set the shapes, call '
---> 98       '`model.build(input_shape)`.'.format(model))
     99 
    100 

ValueError: Model <__main__.MovieLensModel object at 0x7f727eec7690> cannot be saved because the input shapes have not been set. Usually, input shapes are automatically determined from calling `.fit()` or `.predict()`. To manually set the shapes, call `model.build(input_shape)`.

This is reproducible. See https://colab.research.google.com/drive/1yktbZXJUb__VAqWvrCH1h0xgJt3XR7Wc?usp=sharing

How can I fix this?

Start with the TensorFlow Recommenders quickstart example and reproduce the failure in the linked Colab notebook. Inspect the MovieLensModel call path around model.save('my_model') and the input-shape warning. Done means the quickstart model saves successfully without the reported ValueError.