diff --git a/tianshou/core/mcts/mcts.py b/tianshou/core/mcts/mcts.py
index 7edac97..8bb5f06 100644
--- a/tianshou/core/mcts/mcts.py
+++ b/tianshou/core/mcts/mcts.py
@@ -73,7 +73,7 @@ class UCTNode(MCTSNode):
     def valid_mask(self, simulator):
         # let all invalid actions be illeagel in mcts
         if self.mask is None:
-            self.mask = simulator.simulate_is_valid_list(self.state, range(self.action_num - 1))
+            self.mask = simulator.simulate_is_valid_list(self.state, range(self.action_num))
         self.ucb[self.mask] = -float("Inf")