TypeError: '<' not supported between instances of 'NoneType' and 'float' while using callbacks to stop training after a certain mae

Code for W4 Time Series Notebook on using a callback to stop training after a certain mae model: LSTM

class myCallback(tf.keras.callbacks.Callback):
  def on_epoch_end(self, epoch, logs={}):
    '''
    Halts the training when a certain metric is met

    Args:
      epoch (integer) - index of epoch (required but unused in the function definition below)
      logs (dict) - metric results from the training epoch
    '''

    # Check the validation set MAE
    if(logs.get('val_mae') < 5.2):

      # Stop if threshold is met
      print("\nRequired val MAE is met so cancelling training!")
      self.model.stop_training = True

# Instantiate the class
callbacks = myCallback()

returns the error 'TypeError: '<' not supported between instances of 'NoneType' and 'float' on line ' if(logs.get('val_mae') < 5.2):'

Any help on the above will be appreciated!