K.gradients(loss, input_img)[0] return “None”. (Keras CNN visualization with tensorflow backend)

Question

I have CNN models trained using Keras with Tensorflow backend. And I want to visualize my CNN filters with this tutorial: https://blog.keras.io/how-convolutional-neural-networks

Answer 1

I too had faced the same error @Jexus. For me the problem was:

loss variable was None object. I had used

loss.assign_add(....)

instead of

loss = loss + .....

After changing that as mentioned, the loss was returning a tensor and hence

grads = K.gradients(loss, model.input)[0]

wasn't returning None.