padding zero vector not trainable via scatter_update

scatter_update.py

import tensorflow as tf

g = tf.Graph()
with g.as_default():
    a = tf.Variable(initial_value=[[1, 2, 4, 0],[2, 4, 5, 8]])
    b = tf.scatter_update(a, [0], [[0, 0, 0, 0]])

with tf.Session(graph=g) as sess:
   sess.run(tf.initialize_all_variables())
   print(sess.run(a))
   print(sess.run(b))
   print(sess.run(a))
  
  '''
[[1 2 4 0]
[2 4 5 8]]
[[0 0 0 0]
[2 4 5 8]]
[[0 0 0 0]
[2 4 5 8]]
 '''