diff --git a/src/Debug/EcsDebugUtility.cs b/src/Debug/EcsDebugUtility.cs
index 82a36f3..03f2511 100644
--- a/src/Debug/EcsDebugUtility.cs
+++ b/src/Debug/EcsDebugUtility.cs
@@ -305,7 +305,6 @@ namespace DCFApixels.DragonECS
             return obj is IEcsTypeMetaProvider intr ? intr.MetaSource : obj;
         }
         #endregion
-
     }
 
     public interface ITypeMeta
diff --git a/src/Debug/MetaAttributes/MetaGroupAttribute.cs b/src/Debug/MetaAttributes/MetaGroupAttribute.cs
index 0e693d1..332ab92 100644
--- a/src/Debug/MetaAttributes/MetaGroupAttribute.cs
+++ b/src/Debug/MetaAttributes/MetaGroupAttribute.cs
@@ -33,7 +33,7 @@ namespace DCFApixels.DragonECS
         {
             if (string.IsNullOrEmpty(name))
             {
-                Name = string.Empty; 
+                Name = string.Empty;
                 return;
             }
             name = name.Replace('\\', '/');
diff --git a/src/EcsWorld.cs b/src/EcsWorld.cs
index ebe9438..3e1ebf2 100644
--- a/src/EcsWorld.cs
+++ b/src/EcsWorld.cs
@@ -8,25 +8,6 @@ namespace DCFApixels.DragonECS
 {
     public partial class EcsWorld : IEntityStorage
     {
-        [StructLayout(LayoutKind.Explicit, Pack = 4, Size = 4 * 3)]
-        private struct EntitySlot
-        {
-            public static readonly EntitySlot Empty = new EntitySlot(SLEEPING_GEN_FLAG, 0, false);
-            [FieldOffset(0)]
-            public short gen;
-            [FieldOffset(2)]
-            public short componentsCount;
-            [FieldOffset(4)]
-            public bool isUsed;
-            //[FieldOffset(5)]
-            //public bool isLocked;
-            public EntitySlot(short gen, short componentsCount, bool isUsed)
-            {
-                this.gen = gen;
-                this.componentsCount = componentsCount;
-                this.isUsed = isUsed;
-            }
-        }
         public readonly short id;
         private IEcsWorldConfig _config;
 
@@ -120,9 +101,14 @@ namespace DCFApixels.DragonECS
             {
                 config = EcsWorldConfig.Empty;
             }
+            bool nullWorld = this is NullWorld;
+            if(nullWorld == false && worldID == NULL_WORLD_ID)
+            {
+                EcsDebug.PrintWarning($"The world identifier cannot be {NULL_WORLD_ID}");
+            }
             _config = config;
 
-            if (worldID < 0)
+            if (worldID < 0 || (worldID == NULL_WORLD_ID && nullWorld == false))
             {
                 worldID = (short)_worldIdDispenser.UseFree();
             }
@@ -686,6 +672,28 @@ namespace DCFApixels.DragonECS
             return count;
         }
         #endregion
+
+        #region EntitySlot
+        [StructLayout(LayoutKind.Explicit, Pack = 4, Size = 4 * 3)]
+        private struct EntitySlot
+        {
+            public static readonly EntitySlot Empty = new EntitySlot(SLEEPING_GEN_FLAG, 0, false);
+            [FieldOffset(0)]
+            public short gen;
+            [FieldOffset(2)]
+            public short componentsCount;
+            [FieldOffset(4)]
+            public bool isUsed;
+            //[FieldOffset(5)]
+            //public bool isLocked;
+            public EntitySlot(short gen, short componentsCount, bool isUsed)
+            {
+                this.gen = gen;
+                this.componentsCount = componentsCount;
+                this.isUsed = isUsed;
+            }
+        }
+        #endregion
     }
 
     #region Callbacks Interface
diff --git a/src/EcsWorld.static.cs b/src/EcsWorld.static.cs
index c200c99..c1145cf 100644
--- a/src/EcsWorld.static.cs
+++ b/src/EcsWorld.static.cs
@@ -16,6 +16,8 @@ namespace DCFApixels.DragonECS
     }
     public partial class EcsWorld
     {
+        private const short NULL_WORLD_ID = 0;
+
         private const short GEN_MASK = 0x7fff;
         private const short SLEEPING_GEN_FLAG = short.MinValue;
         private const int DEL_ENT_BUFFER_SIZE_OFFSET = 5;
@@ -29,7 +31,7 @@ namespace DCFApixels.DragonECS
 
         static EcsWorld()
         {
-            _worlds[0] = new NullWorld();
+            _worlds[NULL_WORLD_ID] = new NullWorld();
         }
         private static void ReleaseData(int worldID)
         {
@@ -78,8 +80,7 @@ namespace DCFApixels.DragonECS
             public static ref T GetForWorldUnchecked(int worldID)
             {
 #if (DEBUG && !DISABLE_DEBUG)
-                if (_mapping[worldID] <= 0)
-                    throw new Exception();
+                if (_mapping[worldID] <= 0) { Throw.UndefinedException(); }
 #endif
                 return ref _items[_mapping[worldID]];
             }