diff --git a/Source/Map/MapSet.cs b/Source/Map/MapSet.cs
index 008c6aa2..85c02095 100644
--- a/Source/Map/MapSet.cs
+++ b/Source/Map/MapSet.cs
@@ -575,6 +575,11 @@ namespace CodeImp.DoomBuilder.Map
 			MemoryStream stream = new MemoryStream(512000);
 			SerializerStream serializer = new SerializerStream(stream);
 			
+			// Index the sidedefs
+			int sidedefindex = 0;
+			foreach(Sidedef sd in sidedefs)
+				sd.SerializedIndex = sidedefindex++;
+			
 			// Write private data
 			serializer.wInt(lastsectorindex);
 			serializer.wInt(indexholes.Count);
diff --git a/Source/Map/Sidedef.cs b/Source/Map/Sidedef.cs
index 032d1078..52272d1d 100644
--- a/Source/Map/Sidedef.cs
+++ b/Source/Map/Sidedef.cs
@@ -59,6 +59,9 @@ namespace CodeImp.DoomBuilder.Map
 		private long longtexnamemid;
 		private long longtexnamelow;
 
+		// Clone
+		private int serializedindex;
+		
 		#endregion
 
 		#region ================== Properties
@@ -77,6 +80,7 @@ namespace CodeImp.DoomBuilder.Map
 		public long LongHighTexture { get { return longtexnamehigh; } }
 		public long LongMiddleTexture { get { return longtexnamemid; } }
 		public long LongLowTexture { get { return longtexnamelow; } }
+		internal int SerializedIndex { get { return serializedindex; } set { serializedindex = value; } }
 		
 		#endregion