diff --git a/Entity/System/Room.cs b/Entity/System/Room.cs
index 097ee1e..dab0838 100644
--- a/Entity/System/Room.cs
+++ b/Entity/System/Room.cs
@@ -5,11 +5,12 @@ namespace Milimoe.FunGame.Core.Entity
 {
     public class Room : BaseEntity
     {
+        public static readonly Room Empty = new();
         public override long Id { get => base.Id ; set => base.Id = value; }
         public string Roomid { get; set; } = "-1";
         public DateTime CreateTime { get; set; } = General.DefaultTime;
         public Dictionary<string, User> Players { get; set; } = new();
-        public User? RoomMaster { get; set; }
+        public User RoomMaster { get; set; } = General.UnknownUserInstance;
         public RoomType RoomType { get; set; }
         public RoomState RoomState { get; set; }
         public bool HasPass => Password.Trim() != "";
@@ -26,7 +27,7 @@ namespace Milimoe.FunGame.Core.Entity
             this.Id = Id;
             this.Roomid = Roomid;
             this.CreateTime = CreateTime ?? General.DefaultTime;
-            this.RoomMaster = RoomMaster;
+            this.RoomMaster = RoomMaster ?? General.UnknownUserInstance;
             this.RoomType = RoomType;
             this.RoomState = RoomState;
             this.Password = Password;
diff --git a/Entity/User/User.cs b/Entity/User/User.cs
index 6f8cd37..1912131 100644
--- a/Entity/User/User.cs
+++ b/Entity/User/User.cs
@@ -7,6 +7,7 @@ namespace Milimoe.FunGame.Core.Entity
 {
     public class User : BaseEntity
     {
+        public static readonly User Empty = new();
         public override Guid Guid { get; set; } = Guid.NewGuid();
         public override long Id { get; set; }
         public string Username { get; set; } = "";
diff --git a/Library/Common/JsonConverter/RoomConverter.cs b/Library/Common/JsonConverter/RoomConverter.cs
index 091a34a..58ff85c 100644
--- a/Library/Common/JsonConverter/RoomConverter.cs
+++ b/Library/Common/JsonConverter/RoomConverter.cs
@@ -47,7 +47,7 @@ namespace Milimoe.FunGame.Core.Library.Common.JsonConverter
 
                         case RoomQuery.Column_RoomMaster:
                             string master = reader.GetString() ?? "";
-                            room.RoomMaster = JsonSerializer.Deserialize<User>(master, options);
+                            room.RoomMaster = JsonSerializer.Deserialize<User>(master, options) ?? General.UnknownUserInstance;
                             break;
 
                         case RoomQuery.Column_RoomType: