ask an question- ''NoneType' object has no attribute 'pred''

[zwvivid]

Hi, when I implament the idea "Input Gradient Regularization" , I meet a problem that ''NoneType' object has no attribute 'pred'' . I think it is because of double backpropagation. Do you konw how to solve this problem? Thank you very much!

my code is simplified as follows:
.......
image_batch = tf.placeholder(tf.float32, (None, IMG_W, IMG_H,IMG_D), name='image_batch')
label_batch = tf.placeholder(tf.int32, (None,), name='label_batch')
y_conv = model(image_batch)
cross = loss(y_conv ,label_batch)
grads = tf.gradients(cross,image_batch )
input_loss = tf.nn.l2_loss(grads[0])
optimizer = tf.train.MomentumOptimizer(learning_rate = learning_rate,momentum=0.9)
train_op = optimizer.minimize(loss)
....
_ = sess.run(train_op,feed_dict={image_batch:train_batch1,label_batch:train_label_batch1})

ERROR:
train_op = optimizer.minimize(loss)
File "/share/apps/python/lib/python2.7/site-packages/tensorflow/python/training/optimizer.py", line 315, in minimize
grad_loss=grad_loss)
File "/share/apps/python/lib/python2.7/site-packages/tensorflow/python/training/optimizer.py", line 386, in compute_gradients
colocate_gradients_with_ops=colocate_gradients_with_ops)
File "/share/apps/python/lib/python2.7/site-packages/tensorflow/python/ops/gradients_impl.py", line 551, in gradients
out_grads[i] = control_flow_ops.ZerosLikeOutsideLoop(op, i)
File "/share/apps/python/lib/python2.7/site-packages/tensorflow/python/ops/control_flow_ops.py", line 1305, in ZerosLikeOutsideLoop
pred = op_ctxt.pred
AttributeError: 'NoneType' object has no attribute 'pred'