请问 alpha zero训练时，出现如下错误 是什么原因啊

[theorynice]

Game - 104 | data length - 21
kl - nan | lr_multiplier - 2.25 | loss - nan

Traceback (most recent call last):
File "train.py", line 125, in
t.run()
File "train.py", line 62, in run
print(f"Game - {step} | data length - {self.sample(game.play())}")
File "D:\theory\courses\Machine_Learning\torch-light\alpha-zero\game.py", line 152, in play
pi, next_node = mc.search(self.board, node, temperature=1)
TypeError: 'NoneType' object is not iterable