diff --git a/generator/.DevConfigs/66b6150c-da38-4673-9313-83bf9e38a90e.json b/generator/.DevConfigs/66b6150c-da38-4673-9313-83bf9e38a90e.json
new file mode 100644
index 000000000000..1254453185d1
--- /dev/null
+++ b/generator/.DevConfigs/66b6150c-da38-4673-9313-83bf9e38a90e.json
@@ -0,0 +1,9 @@
+{
+  "services": [
+    {
+      "serviceName": "CloudFormation",
+      "type": "patch",
+      "changeLogMessages": [ "Fixed an issue where NullReferenceException was thrown while invoking AmazonCloudFormationClient.UpdateStackAsync()." ]
+    }
+  ]
+}
\ No newline at end of file
diff --git a/sdk/src/Services/CloudFormation/Custom/Internal/ProcessRequestHandler.cs b/sdk/src/Services/CloudFormation/Custom/Internal/ProcessRequestHandler.cs
index 361020a7ffec..f13047362273 100644
--- a/sdk/src/Services/CloudFormation/Custom/Internal/ProcessRequestHandler.cs
+++ b/sdk/src/Services/CloudFormation/Custom/Internal/ProcessRequestHandler.cs
@@ -68,13 +68,22 @@ protected virtual void PreInvoke(IExecutionContext executionContext)
             if (updateStackRequest != null)
             {
                 var arns = updateStackRequest.NotificationARNs;
-                bool arnsAutoConstructed = arns is AutoConstructedList<string>;
 
-                // if there are no NotificationARNs and the list was created by user (type is NOT AutoConstructed)
-                // only then pass empty param
-                if (arns.Count == 0 && !arnsAutoConstructed)
+                if (arns != null)
                 {
-                    request.Parameters.Add("NotificationARNs", "");
+                    // if there are no NotificationARNs and the list was created by user (type is NOT AutoConstructed)
+                    // only then pass empty param
+                    if (arns.Count == 0 && !(arns is AutoConstructedList<string>))
+                    {
+                        if (request.Parameters.ContainsKey("NotificationARNs"))
+                        {
+                            request.Parameters["NotificationARNs"] = string.Empty;
+                        }
+                        else
+                        {
+                            request.Parameters.Add("NotificationARNs", string.Empty);
+                        }
+                    }
                 }
             }
         }
diff --git a/sdk/test/Services/CloudFormation/IntegrationTests/CloudFormation.cs b/sdk/test/Services/CloudFormation/IntegrationTests/CloudFormation.cs
index 5eade6eab7bc..b433a95dbd15 100644
--- a/sdk/test/Services/CloudFormation/IntegrationTests/CloudFormation.cs
+++ b/sdk/test/Services/CloudFormation/IntegrationTests/CloudFormation.cs
@@ -218,6 +218,86 @@ public void TestCreateStack()
             }
         }
 
+        [TestMethod]
+        [TestCategory("CloudFormation")]
+        public void TestUpdateStack()
+        {
+            string stackName = "test-stack-" + DateTime.UtcNow.Ticks;
+            try
+            {
+                CreateStackRequest createRequest = new CreateStackRequest
+                {
+                    StackName = stackName,
+                    TemplateBody = TEMPLATE_TEXT,
+                    Parameters = new List<Parameter>
+                    {
+                        new Parameter
+                        {
+                            ParameterKey = "TopicName",
+                            ParameterValue = "MyTopic" + DateTime.UtcNow.Ticks
+                        },
+                        new Parameter
+                        {
+                            ParameterKey = "QueueName",
+                            ParameterValue = "MyQueue" + DateTime.UtcNow.Ticks
+                        }
+                    }
+                };
+
+                Client.CreateStack(createRequest);
+                WaitTillStackNotInProcess(stackName);
+
+                UpdateStackRequest updateStackRequest = new UpdateStackRequest
+                {
+                    StackName = stackName,
+                    TemplateBody = TEMPLATE_TEXT,
+                    Parameters = new List<Parameter>
+                    {
+                        new Parameter
+                        {
+                            ParameterKey = "TopicName",
+                            ParameterValue = "MyTopic" + DateTime.UtcNow.Ticks
+                        },
+                        new Parameter
+                        {
+                            ParameterKey = "QueueName",
+                            ParameterValue = "MyQueue" + DateTime.UtcNow.Ticks
+                        }
+                    }
+                };
+
+                Client.UpdateStack(updateStackRequest);
+                WaitTillStackNotInProcess(stackName);
+
+                updateStackRequest = new UpdateStackRequest
+                {
+                    StackName = stackName,
+                    TemplateBody = TEMPLATE_TEXT,
+                    Parameters = new List<Parameter>
+                    {
+                        new Parameter
+                        {
+                            ParameterKey = "TopicName",
+                            ParameterValue = "MyTopic" + DateTime.UtcNow.Ticks
+                        },
+                        new Parameter
+                        {
+                            ParameterKey = "QueueName",
+                            ParameterValue = "MyQueue" + DateTime.UtcNow.Ticks
+                        }
+                    },
+                    NotificationARNs = new List<string>()
+                };
+                Client.UpdateStack(updateStackRequest);
+                WaitTillStackNotInProcess(stackName);
+            }
+            finally
+            {
+                WaitTillStackNotInProcess(stackName);
+                Client.DeleteStack(new DeleteStackRequest() { StackName = stackName });
+            }
+        }
+
         static void WaitTillStackNotInProcess(string stackname)
         {
             DescribeStacksResponse response = null;