diff --git a/.github/workflows/ReleaseNotes.md b/.github/workflows/ReleaseNotes.md
index 4983fc8d..087b683a 100644
--- a/.github/workflows/ReleaseNotes.md
+++ b/.github/workflows/ReleaseNotes.md
@@ -1 +1,2 @@
+* [Core] Optimized packet serialization of PUBACK and PUBREC packets for protocol version 5.0.0 (#1939, thanks to @Y-Sindo).
 * [Core] The package inspector is now fully async (#1941).
diff --git a/Source/MQTTnet/Formatter/V5/MqttV5PacketEncoder.cs b/Source/MQTTnet/Formatter/V5/MqttV5PacketEncoder.cs
index 8023c9c0..8e6804f3 100644
--- a/Source/MQTTnet/Formatter/V5/MqttV5PacketEncoder.cs
+++ b/Source/MQTTnet/Formatter/V5/MqttV5PacketEncoder.cs
@@ -279,7 +279,7 @@ namespace MQTTnet.Formatter.V5
             _propertiesWriter.WriteReasonString(packet.ReasonString);
             _propertiesWriter.WriteUserProperties(packet.UserProperties);
 
-            if (_bufferWriter.Length > 0 || packet.ReasonCode != MqttPubAckReasonCode.Success)
+            if (_propertiesWriter.Length > 0 || packet.ReasonCode != MqttPubAckReasonCode.Success)
             {
                 _bufferWriter.WriteByte((byte)packet.ReasonCode);
                 _propertiesWriter.WriteTo(_bufferWriter);
@@ -375,7 +375,7 @@ namespace MQTTnet.Formatter.V5
 
             _bufferWriter.WriteTwoByteInteger(packet.PacketIdentifier);
 
-            if (_bufferWriter.Length > 0 || packet.ReasonCode != MqttPubRecReasonCode.Success)
+            if (_propertiesWriter.Length > 0 || packet.ReasonCode != MqttPubRecReasonCode.Success)
             {
                 _bufferWriter.WriteByte((byte)packet.ReasonCode);
                 _propertiesWriter.WriteTo(_bufferWriter);
@@ -534,4 +534,4 @@ namespace MQTTnet.Formatter.V5
             }
         }
     }
-}
\ No newline at end of file
+}