diff --git a/Tests/MQTTnet.Core.Tests/Mockups/TestEnvironment.cs b/Tests/MQTTnet.Core.Tests/Mockups/TestEnvironment.cs
index 3fb399e9..5800bf43 100644
--- a/Tests/MQTTnet.Core.Tests/Mockups/TestEnvironment.cs
+++ b/Tests/MQTTnet.Core.Tests/Mockups/TestEnvironment.cs
@@ -66,10 +66,13 @@ namespace MQTTnet.Tests.Mockups
 
         public IMqttClient CreateClient()
         {
-            var client = _mqttFactory.CreateMqttClient(ClientLogger);
-            _clients.Add(client);
+            lock (_clients)
+            {
+                var client = _mqttFactory.CreateMqttClient(ClientLogger);
+                _clients.Add(client);
 
-            return new TestClientWrapper(client, TestContext);
+                return new TestClientWrapper(client, TestContext);
+            }
         }
 
         public Task<IMqttServer> StartServerAsync()
diff --git a/Tests/MQTTnet.Core.Tests/Server_Tests.cs b/Tests/MQTTnet.Core.Tests/Server_Tests.cs
index 16db377e..71c819d1 100644
--- a/Tests/MQTTnet.Core.Tests/Server_Tests.cs
+++ b/Tests/MQTTnet.Core.Tests/Server_Tests.cs
@@ -646,11 +646,11 @@ namespace MQTTnet.Tests
                             {
                                 // Clear retained message.
                                 await client.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("r" + i2)
-                                    .WithPayload(new byte[0]).WithRetainFlag().Build());
+                                    .WithPayload(new byte[0]).WithRetainFlag().WithQualityOfServiceLevel(1).Build());
 
                                 // Set retained message.
                                 await client.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("r" + i2)
-                                    .WithPayload("value").WithRetainFlag().Build());
+                                    .WithPayload("value").WithRetainFlag().WithQualityOfServiceLevel(1).Build());
 
                                 await client.DisconnectAsync();
                             }