diff --git a/Solver/ActionSet.cs b/Solver/ActionSet.cs
index c7aaf34..248ba13 100644
--- a/Solver/ActionSet.cs
+++ b/Solver/ActionSet.cs
@@ -16,7 +16,7 @@ public struct ActionSet
     private static int FromAction(ActionType action)
     {
         var ret = Simulator.AcceptedActionsLUT[(byte)action];
-        if (ret == 0)
+        if (ret == -1)
             throw new ArgumentOutOfRangeException(nameof(action), action, $"Action {action} is unsupported in {nameof(ActionSet)}.");
         return ret;
     }
diff --git a/Solver/Simulator.cs b/Solver/Simulator.cs
index b3f6f60..23ab77c 100644
--- a/Solver/Simulator.cs
+++ b/Solver/Simulator.cs
@@ -63,6 +63,8 @@ public Simulator(SimulationState state, int maxStepCount) : base(state)
     static Simulator()
     {
         AcceptedActionsLUT = new int[Enum.GetValues<ActionType>().Length];
+        for (var i = 0; i < AcceptedActionsLUT.Length; i++)
+            AcceptedActionsLUT[i] = -1;
         for (var i = 0; i < AcceptedActions.Length; i++)
             AcceptedActionsLUT[(byte)AcceptedActions[i]] = i;
     }