Why does Tensorflow Reshape tf.reshape() break the flow of gradients?

I am creating a tf.Variable() and then create a simple function using that variable, then I flatten the original variable using tf.reshape() and then I take the tf.gradients() between the function and the flattened variable. Why does that return [None].

var = tf.Variable(np.ones((5,5)), dtype = tf.float32)
f = tf.reduce_sum(tf.reduce_sum(tf.square(var)))
var_f = tf.reshape(var, [-1])
print tf.gradients(f,var_f)

The above codeblock when executed returns [None]. Is this a bug? Please Help!