diff --git a/Build/MQTTnet.AspNetCore.nuspec b/Build/MQTTnet.AspNetCore.nuspec
index 9cf4005..3071733 100644
--- a/Build/MQTTnet.AspNetCore.nuspec
+++ b/Build/MQTTnet.AspNetCore.nuspec
@@ -2,7 +2,7 @@
 <package >
   <metadata>
     <id>MQTTnet.AspNetCore</id>
-    <version>2.5.0</version>
+    <version>2.5.1</version>
     <authors>Christian Kratky</authors>
     <owners>Christian Kratky</owners>
     <licenseUrl>https://github.com/chkr1011/MQTTnet/blob/master/LICENSE</licenseUrl>
@@ -10,13 +10,13 @@
     <iconUrl>https://raw.githubusercontent.com/chkr1011/MQTTnet/master/Images/Logo_128x128.png</iconUrl>
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <description>This is a support library to integrate MQTTnet into AspNetCore.</description>
-    <releaseNotes>initial version
+    <releaseNotes>* Updated to MQTTnet 2.5.1.
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2017</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin</tags> 
     <dependencies>
       <group targetFramework="netstandard2.0">
-        <dependency id="MQTTnet" version="2.5" />
+        <dependency id="MQTTnet" version="2.5.1" />
       </group>
     </dependencies>
   </metadata>
diff --git a/Build/MQTTnet.nuspec b/Build/MQTTnet.nuspec
index 771f536..348ca79 100644
--- a/Build/MQTTnet.nuspec
+++ b/Build/MQTTnet.nuspec
@@ -10,14 +10,15 @@
     <iconUrl>https://raw.githubusercontent.com/chkr1011/MQTTnet/master/Images/Logo_128x128.png</iconUrl>
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <description>MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker).</description>
-    <releaseNotes>* 
+    <releaseNotes>* Fixed several library reference issues.
+* [Client] Fixed issue in _MqttClientOptionsBuilder_ when using no certificates (Thanks to 1iveowl).
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2017</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin</tags> 
     <dependencies>
 
       <group targetFramework="netstandard1.3">
-        <dependency id="NETStandard.Library" version="1.6.1" />
+        <dependency id="NETStandard.Library" version="1.3" />
         <dependency id="Microsoft.Extensions.DependencyInjection" version="1.1.1" />
         <dependency id="Microsoft.Extensions.Logging" version="1.1.2" />
         <dependency id="Microsoft.Extensions.Options" version="1.1.2" />
@@ -45,7 +46,7 @@
         <dependency id="Microsoft.Extensions.Options" version="1.1.2" />
       </group>
         
-      <group targetFramework="net451">
+      <group targetFramework="net452">
         <dependency id="Microsoft.Extensions.DependencyInjection" version="1.1.1" />
         <dependency id="Microsoft.Extensions.Logging" version="1.1.2" />
         <dependency id="Microsoft.Extensions.Options" version="1.1.2" />
diff --git a/MQTTnet.Core/Client/MqttClientOptionsBuilder.cs b/MQTTnet.Core/Client/MqttClientOptionsBuilder.cs
index 29a2d64..fb18cc0 100644
--- a/MQTTnet.Core/Client/MqttClientOptionsBuilder.cs
+++ b/MQTTnet.Core/Client/MqttClientOptionsBuilder.cs
@@ -98,7 +98,7 @@ namespace MQTTnet.Core.Client
                 AllowUntrustedCertificates = allowUntrustedCertificates,
                 IgnoreCertificateChainErrors = ignoreCertificateChainErrors,
                 IgnoreCertificateRevocationErrors = ignoreCertificateRevocationErrors,
-                Certificates = certificates.ToList()
+                Certificates = certificates?.ToList()
             };
 
             return this;
diff --git a/MQTTnet.Core/Server/MqttClientPendingMessagesQueue.cs b/MQTTnet.Core/Server/MqttClientPendingMessagesQueue.cs
index 6b370db..1e0fc76 100644
--- a/MQTTnet.Core/Server/MqttClientPendingMessagesQueue.cs
+++ b/MQTTnet.Core/Server/MqttClientPendingMessagesQueue.cs
@@ -28,6 +28,11 @@ namespace MQTTnet.Core.Server
         {
             if (adapter == null) throw new ArgumentNullException(nameof(adapter));
 
+            if (cancellationToken.IsCancellationRequested)
+            {
+                return;
+            }
+
             Task.Run(async () => await SendPendingPublishPacketsAsync(adapter, cancellationToken), cancellationToken).ConfigureAwait(false);
         }
 
diff --git a/MQTTnet.Core/Server/MqttClientSession.cs b/MQTTnet.Core/Server/MqttClientSession.cs
index cbdfc73..93b422f 100644
--- a/MQTTnet.Core/Server/MqttClientSession.cs
+++ b/MQTTnet.Core/Server/MqttClientSession.cs
@@ -60,12 +60,14 @@ namespace MQTTnet.Core.Server
 
             try
             {
+                var cancellationTokenSource = new CancellationTokenSource();
+
                 _willMessage = willMessage;
                 _adapter = adapter;
-                _cancellationTokenSource = new CancellationTokenSource();
+                _cancellationTokenSource = cancellationTokenSource;
 
-                _pendingMessagesQueue.Start(adapter, _cancellationTokenSource.Token);
-                await ReceivePacketsAsync(adapter, _cancellationTokenSource.Token).ConfigureAwait(false);
+                _pendingMessagesQueue.Start(adapter, cancellationTokenSource.Token);
+                await ReceivePacketsAsync(adapter, cancellationTokenSource.Token).ConfigureAwait(false);
             }
             catch (OperationCanceledException)
             {