Validation_accuracy not showing up

Hi everyone. Does anyone have experienced below?

I ran this cell.

Get the untrained model

model = create_model()

Train the model

Note that this may take some time.

history = model.fit(train_generator,
epochs=15,
verbose=1,
validation_data=validation_generator)

After this, I got accuracy being calculated. However, there is no val_accuracy, which make me fail to plot it next.

176/176 [==============================] - 169s 882ms/step - loss: 0.6846 - accuracy: 0.5616 Epoch 2/15 176/176 [==============================] - 156s 885ms/step - loss: 0.6540 - accuracy: 0.6081 Epoch 3/15 176/176 [==============================] - 156s 887ms/step - loss: 0.6177 - accuracy: 0.6570 Epoch 4/15 176/176 [==============================] - 155s 879ms/step - loss: 0.5906 - accuracy: 0.6837 Epoch 5/15 176/176 [==============================] - 156s 885ms/step - loss: 0.5730 - accuracy: 0.7003 Epoch 6/15 176/176 [==============================] - 155s 878ms/step - loss: 0.5537 - accuracy: 0.7180 Epoch 7/15 176/176 [==============================] - 156s 884ms/step - loss: 0.5419 - accuracy: 0.7279 Epoch 8/15 176/176 [==============================] - 155s 878ms/step - loss: 0.5239 - accuracy: 0.7428 Epoch 9/15 176/176 [==============================] - 155s 879ms/step - loss: 0.5042 - accuracy: 0.7526 Epoch 10/15 176/176 [==============================] - 155s 878ms/step - loss: 0.4870 - accuracy: 0.7650 Epoch 11/15 140/176 [======================>…] - ETA: 31s - loss: 0.4635 - accuracy: 0.7764

validation_generator should not be None when invoking model.fit