diff --git a/src/NosCore.Packets/Serializer.cs b/src/NosCore.Packets/Serializer.cs
index e53ce85..4d23054 100644
--- a/src/NosCore.Packets/Serializer.cs
+++ b/src/NosCore.Packets/Serializer.cs
@@ -273,7 +273,7 @@ private Expression PacketSerializer(Expression injectedPacket, PacketIndexAttrib
 
             return Expression.Condition(
                 Expression.Equal(specificTypeExpression, Expression.Constant(null, typeof(object))),
-                Expression.Constant(indexAttr.IsOptional ? null : "-1", typeof(object)),
+                Expression.Constant(indexAttr.IsOptional ? null : $"{discriminator}-1", typeof(object)),
                 Expression.Convert(propExp, typeof(object))
             );
         }
diff --git a/test/NosCore.Packets.Tests/SerializerTest.cs b/test/NosCore.Packets.Tests/SerializerTest.cs
index db9843f..3c2a5c7 100644
--- a/test/NosCore.Packets.Tests/SerializerTest.cs
+++ b/test/NosCore.Packets.Tests/SerializerTest.cs
@@ -80,6 +80,7 @@ public class SerializationTests
                 typeof(MallPacket),
                 typeof(FtptPacket),
                 typeof(ScpIndicatorPacket),
+                typeof(ScnPacket),
                 typeof(EsfPacket),
                 typeof(SopenPacket),
                 typeof(StbmPacket),
@@ -518,6 +519,26 @@ public void SerializeWithNullFirstParam()
                 packet);
         }
 
+        [TestMethod]
+        public void SerializeWithNullSubPacketKeepsTheSeparator()
+        {
+            var packet = Serializer.Serialize(new ScnPacket
+            {
+                PetId = 1,
+                NpcMonsterVNum = 319,
+                TransportId = 26719,
+                Level = 50,
+                Loyalty = 1000,
+                Experience = 1536,
+                WeaponInstanceDetails = new ScnPacket.ScEquipmentDetails { ItemId = 990 },
+                Name = "Kliff",
+                MorphId = -1
+            });
+
+            Assert.IsTrue(packet.StartsWith("sc_n 1 319 26719 50 1000 1536 990.0.0 -1 -1 -1 "),
+                $"the -1 of a null sub-packet must not be glued to the field before it: {packet}");
+        }
+
         [TestMethod]
         public void SerializeWithSpecialSeparator()
         {