diff --git a/tianshou/policy/base.py b/tianshou/policy/base.py
index 19bc58dea..2cd0fbd7c 100644
--- a/tianshou/policy/base.py
+++ b/tianshou/policy/base.py
@@ -138,7 +138,8 @@ def map_action(self, act: Union[Batch, np.ndarray]) -> Union[Batch, np.ndarray]:
         :return: action in the same form of input "act" but remap to the target action
             space.
         """
-        if isinstance(self.action_space, gym.spaces.Box):
+        if isinstance(self.action_space, gym.spaces.Box) and \
+                isinstance(act, np.ndarray):
             # currently this action mapping only supports np.ndarray action
             if self.action_bound_method == "clip":
                 act = np.clip(act, -1, 1)