'tensorflow' has no attribute 'sub'

Traceback (most recent call last):
  File "interactive.py", line 10, in <module>
    sub = tf.sub(x,a)
AttributeError: module 'tensorflow' has no attribute 'sub'

# -*- coding: utf-8 -*-
import tensorflow as tf
sess = tf.InteractiveSession()

x = tf.Variable([1.0, 2.0])
a = tf.constant([3.0, 3.0])

x.initializer.run()

sub = tf.subtract(x,a)
print(sub.eval())