diff --git a/physbo/search/discrete_multi/policy.py b/physbo/search/discrete_multi/policy.py
index 877201f4..495bcb64 100644
--- a/physbo/search/discrete_multi/policy.py
+++ b/physbo/search/discrete_multi/policy.py
@@ -58,7 +58,7 @@ def __init__(
                 msg = "ERROR: len(initial_data[0]) != initial_data[1].shape[0]"
                 raise RuntimeError(msg)
             self.write(actions, fs)
-            self.actions = sorted(list(set(self.actions) - set(actions)))
+            self.actions = np.array(sorted(list(set(self.actions) - set(actions))))
 
         if comm is None:
             self.mpicomm = None