diff --git a/composer.json b/composer.json
index 94f04e95f..69f5e6b0c 100755
--- a/composer.json
+++ b/composer.json
@@ -11,7 +11,7 @@
     "guzzlehttp/psr7": "^1.2",
     "monolog/monolog": "^1.17.1 || ^2.0.0",
     "phpdocumentor/reflection-docblock": "^4.0.0 || ^3.0.3",
-    "symfony/serializer": "^4.4.0"
+    "symfony/serializer": "^4.4.0 || ^5.0.0"
   },
   "require-dev": {
     "phpunit/phpunit": "^7.5",
diff --git a/src/Google/AdsApi/AdWords/AdWordsNormalizer.php b/src/Google/AdsApi/AdWords/AdWordsNormalizer.php
index d6275c624..7970503ae 100755
--- a/src/Google/AdsApi/AdWords/AdWordsNormalizer.php
+++ b/src/Google/AdsApi/AdWords/AdWordsNormalizer.php
@@ -73,6 +73,7 @@ final class AdWordsNormalizer extends GetSetMethodNormalizer
 {
 
     private $docBlockFactory;
+    protected $callbacks = []; // not private for symfony 4 compatibility
 
     /**
      * @see GetSetMethodNormalizer::__construct()
@@ -110,7 +111,7 @@ public function normalize($object, $format = null, array $context = [])
             $attributeValue = $reflMethod->invoke($object);
 
             if ($attributeValue !== null) {
-                if (array_key_exists('normalize', $this->callbacks)) {
+                if (isset($this->callbacks['normalize'])) {
                     $attributeValue = call_user_func(
                         $this->callbacks['normalize'],
                         $attributeValue,
@@ -207,7 +208,7 @@ public function denormalize(
                     $this->docBlockFactory->create($getter->getDocComment())
                 );
 
-            if (array_key_exists('denormalize', $this->callbacks)) {
+            if (isset($this->callbacks['denormalize'])) {
                 $value = call_user_func(
                     $this->callbacks['denormalize'],
                     $value,
diff --git a/src/Google/AdsApi/AdWords/BatchJobs/v201809/BatchJobsDelegate.php b/src/Google/AdsApi/AdWords/BatchJobs/v201809/BatchJobsDelegate.php
index 53be0823a..51095f6c8 100755
--- a/src/Google/AdsApi/AdWords/BatchJobs/v201809/BatchJobsDelegate.php
+++ b/src/Google/AdsApi/AdWords/BatchJobs/v201809/BatchJobsDelegate.php
@@ -120,7 +120,7 @@ public function __construct(
         );
         $this->batchJobSerializer = new Serializer(
             [$adWordsNormalizer, new ArrayDenormalizer()],
-            [new XmlEncoder('ns1:mutate')]
+            [new XmlEncoder([XmlEncoder::ROOT_NODE_NAME => 'ns1:mutate'])]
         );
     }