diff --git a/SVSim.EmulatedEntrypoint/Models/Dtos/Responses/ArenaTwoPick/DoMatchingResponseDto.cs b/SVSim.EmulatedEntrypoint/Models/Dtos/Responses/ArenaTwoPick/DoMatchingResponseDto.cs
index 7c3411f..81a73d3 100644
--- a/SVSim.EmulatedEntrypoint/Models/Dtos/Responses/ArenaTwoPick/DoMatchingResponseDto.cs
+++ b/SVSim.EmulatedEntrypoint/Models/Dtos/Responses/ArenaTwoPick/DoMatchingResponseDto.cs
@@ -22,6 +22,13 @@ public sealed class DoMatchingResponseDto
     [JsonPropertyName("node_server_url")] [Key("node_server_url")]
     public string NodeServerUrl { get; set; } = "";
 
+    // Required by the client when matching_state ∈ {3004, 3007, 3011} —
+    // DoMatchingBase.SettingCardMasterId does jsonData["card_master_id"].ToInt()
+    // with no Keys.Contains guard, so omitting it throws KeyNotFoundException.
+    // Value matches what /load/index returns (the "current battle card master").
+    [JsonPropertyName("card_master_id")] [Key("card_master_id")]
+    public int CardMasterId { get; set; } = 1;
+
     [JsonPropertyName("room_param")] [Key("room_param")]
     public string RoomParam { get; set; } = "";
 
diff --git a/SVSim.UnitTests/Controllers/ArenaTwoPickBattleControllerTests.cs b/SVSim.UnitTests/Controllers/ArenaTwoPickBattleControllerTests.cs
index 33c4ea8..a75d1f9 100644
--- a/SVSim.UnitTests/Controllers/ArenaTwoPickBattleControllerTests.cs
+++ b/SVSim.UnitTests/Controllers/ArenaTwoPickBattleControllerTests.cs
@@ -29,5 +29,8 @@ public class ArenaTwoPickBattleControllerTests
         Assert.That(battleId, Is.Not.Null.And.Not.Empty);
         var nodeUrl = root.GetProperty("node_server_url").GetString();
         Assert.That(nodeUrl, Does.StartWith("ws://"));
+        // Required when matching_state ∈ {3004,3007,3011} per
+        // DoMatchingBase.SettingCardMasterId; client throws KeyNotFoundException without it.
+        Assert.That(root.GetProperty("card_master_id").GetInt32(), Is.EqualTo(1));
     }
 }