TypeError: Fetch argument None has invalid type <class 'NoneType'>

使用tensorflow可视化工具tf.summary时出现的错误：TypeError: Fetch argument None has invalid type <class 'NoneType'>

代码：

初始化：
self.merged = tf.summary.merge_all()
self.writer = tf.summary.FileWriter("logs/val", sess.graph)

使用：

test = sess.run(self.merged, {self.input_state: feed_state,
                 self.input_action: feed_action,
                 self.y: feed_y})
self.writer.add_summary(test, 1)


错误原因：

self.merged = tf.summary.merge_all()放在了变量初始化init前面，导致self.merged为NoneType