diff --git a/composer.json b/composer.json
index 5592e2c5..1af2a500 100644
--- a/composer.json
+++ b/composer.json
@@ -12,7 +12,7 @@
     ],
     "require": {
         "php": "^7.4 || ^8.0",
-        "symfony/serializer": "^5.0",
+        "symfony/serializer": "^5.0 || ^6.0",
         "elasticsearch/elasticsearch": "^7.0"
     },
     "require-dev": {
diff --git a/src/SearchEndpoint/AggregationsEndpoint.php b/src/SearchEndpoint/AggregationsEndpoint.php
index 3e035f43..a50a515b 100644
--- a/src/SearchEndpoint/AggregationsEndpoint.php
+++ b/src/SearchEndpoint/AggregationsEndpoint.php
@@ -27,7 +27,7 @@ class AggregationsEndpoint extends AbstractSearchEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = []): array|string|int|float|bool
     {
         $output = [];
         if (count($this->getAll()) > 0) {
diff --git a/src/SearchEndpoint/HighlightEndpoint.php b/src/SearchEndpoint/HighlightEndpoint.php
index aa57b3d7..0549994e 100644
--- a/src/SearchEndpoint/HighlightEndpoint.php
+++ b/src/SearchEndpoint/HighlightEndpoint.php
@@ -37,13 +37,13 @@ class HighlightEndpoint extends AbstractSearchEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = []): array|string|int|float|bool
     {
         if ($this->highlight) {
             return $this->highlight->toArray();
         }
 
-        return null;
+        return false;
     }
 
     /**
diff --git a/src/SearchEndpoint/InnerHitsEndpoint.php b/src/SearchEndpoint/InnerHitsEndpoint.php
index 8428bd88..58e782b2 100644
--- a/src/SearchEndpoint/InnerHitsEndpoint.php
+++ b/src/SearchEndpoint/InnerHitsEndpoint.php
@@ -27,7 +27,7 @@ class InnerHitsEndpoint extends AbstractSearchEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = []): array|string|int|float|bool
     {
         $output = [];
         if (count($this->getAll()) > 0) {
diff --git a/src/SearchEndpoint/PostFilterEndpoint.php b/src/SearchEndpoint/PostFilterEndpoint.php
index de3e263c..d0e9a09f 100644
--- a/src/SearchEndpoint/PostFilterEndpoint.php
+++ b/src/SearchEndpoint/PostFilterEndpoint.php
@@ -26,10 +26,10 @@ class PostFilterEndpoint extends QueryEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = []): array|string|int|float|bool
     {
         if (!$this->getBool()) {
-            return null;
+            return false;
         }
 
         return $this->getBool()->toArray();
diff --git a/src/SearchEndpoint/QueryEndpoint.php b/src/SearchEndpoint/QueryEndpoint.php
index b7626910..17d96c0c 100644
--- a/src/SearchEndpoint/QueryEndpoint.php
+++ b/src/SearchEndpoint/QueryEndpoint.php
@@ -39,7 +39,7 @@ class QueryEndpoint extends AbstractSearchEndpoint implements OrderedNormalizerI
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = []): array|string|int|float|bool
     {
         if (!$this->filtersSet && $this->hasReference('filter_query')) {
             /** @var BuilderInterface $filter */
@@ -49,7 +49,7 @@ public function normalize(NormalizerInterface $normalizer, string $format = null
         }
 
         if (!$this->bool) {
-            return null;
+            return false;
         }
 
         return $this->bool->toArray();
diff --git a/src/SearchEndpoint/SortEndpoint.php b/src/SearchEndpoint/SortEndpoint.php
index cd00bb2f..009de9d8 100644
--- a/src/SearchEndpoint/SortEndpoint.php
+++ b/src/SearchEndpoint/SortEndpoint.php
@@ -26,7 +26,7 @@ class SortEndpoint extends AbstractSearchEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = []): array|string|int|float|bool
     {
         $output = [];
 
diff --git a/src/SearchEndpoint/SuggestEndpoint.php b/src/SearchEndpoint/SuggestEndpoint.php
index 6d292239..733b3efc 100644
--- a/src/SearchEndpoint/SuggestEndpoint.php
+++ b/src/SearchEndpoint/SuggestEndpoint.php
@@ -27,7 +27,7 @@ class SuggestEndpoint extends AbstractSearchEndpoint
     /**
      * {@inheritdoc}
      */
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = []): array|string|int|float|bool
     {
         $output = [];
         if (count($this->getAll()) > 0) {
diff --git a/src/Serializer/Normalizer/CustomReferencedNormalizer.php b/src/Serializer/Normalizer/CustomReferencedNormalizer.php
index fd32a89b..d7a6bd9b 100644
--- a/src/Serializer/Normalizer/CustomReferencedNormalizer.php
+++ b/src/Serializer/Normalizer/CustomReferencedNormalizer.php
@@ -26,7 +26,7 @@ class CustomReferencedNormalizer extends CustomNormalizer
     /**
      * {@inheritdoc}
      */
-    public function normalize($object, string $format = null, array $context = [])
+    public function normalize(mixed $object, string $format = null, array $context = []): array|string|int|float|bool|\ArrayObject|null
     {
         $object->setReferences($this->references);
         $data = parent::normalize($object, $format, $context);
@@ -38,7 +38,7 @@ public function normalize($object, string $format = null, array $context = [])
     /**
      * {@inheritdoc}
      */
-    public function supportsNormalization($data, $format = null)
+    public function supportsNormalization(mixed $data, string $format = null): bool
     {
         return $data instanceof AbstractNormalizable;
     }
diff --git a/src/Serializer/OrderedSerializer.php b/src/Serializer/OrderedSerializer.php
index 9019e43e..7179069f 100644
--- a/src/Serializer/OrderedSerializer.php
+++ b/src/Serializer/OrderedSerializer.php
@@ -22,7 +22,7 @@ class OrderedSerializer extends Serializer
     /**
      * {@inheritdoc}
      */
-    public function normalize($data, $format = null, array $context = [])
+    public function normalize(mixed $data, string $format = null, array $context = []): array|string|int|float|bool|\ArrayObject|null
     {
         return parent::normalize(
             is_array($data) ? $this->order($data) : $data,
@@ -34,7 +34,7 @@ public function normalize($data, $format = null, array $context = [])
     /**
      * {@inheritdoc}
      */
-    public function denormalize($data, $type, $format = null, array $context = [])
+    public function denormalize(mixed $data, string $type, string $format = null, array $context = []): mixed
     {
         return parent::denormalize(
             is_array($data) ? $this->order($data) : $data,
@@ -86,3 +86,4 @@ function ($value) {
         );
     }
 }
+