diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3194ef0..0e7dcf9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,7 @@ Check our main [developer changelog](https://developer.paddle.com/?utm_source=dx
   - `details->lineItems[]->priceId` is now nullable
   - `items[]->priceId` is now nullable
   - `details->lineItems[]->product` can now return `Product` (with `id`) or `TransactionPreviewProduct` (with nullable `id`)
+- Empty custom data array will now serialize to empty JSON object `{}`
 
 ### Added
 - `TransactionsClient::create()` now supports operation items with optional properties:
diff --git a/src/Client.php b/src/Client.php
index 328d259..3bf87de 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -43,6 +43,7 @@
 use Psr\Log\NullLogger;
 use Symfony\Component\Serializer\Encoder\JsonEncoder;
 use Symfony\Component\Serializer\NameConverter\CamelCaseToSnakeCaseNameConverter;
+use Symfony\Component\Serializer\Normalizer\AbstractObjectNormalizer;
 use Symfony\Component\Serializer\Normalizer\BackedEnumNormalizer;
 use Symfony\Component\Serializer\Normalizer\JsonSerializableNormalizer;
 use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;
@@ -177,7 +178,9 @@ private function requestRaw(string $method, string|UriInterface $uri, array|\Jso
         );
 
         if ($payload !== null) {
-            $body = $serializer->serialize($payload, 'json');
+            $body = $serializer->serialize($payload, 'json', [
+                AbstractObjectNormalizer::PRESERVE_EMPTY_OBJECTS => true,
+            ]);
 
             $request = $request->withBody(
                 // Satisfies empty body requests.
diff --git a/src/Entities/Shared/CustomData.php b/src/Entities/Shared/CustomData.php
index 4ba2593..f60433a 100644
--- a/src/Entities/Shared/CustomData.php
+++ b/src/Entities/Shared/CustomData.php
@@ -18,8 +18,10 @@ public function __construct(
     ) {
     }
 
-    public function jsonSerialize(): array|\JsonSerializable
+    public function jsonSerialize(): array|\JsonSerializable|\stdClass
     {
-        return $this->data;
+        return $this->data === []
+            ? (object) []
+            : $this->data;
     }
 }
diff --git a/src/Notifications/Entities/Shared/CustomData.php b/src/Notifications/Entities/Shared/CustomData.php
index 019422c..fcd4b3e 100644
--- a/src/Notifications/Entities/Shared/CustomData.php
+++ b/src/Notifications/Entities/Shared/CustomData.php
@@ -18,8 +18,10 @@ public function __construct(
     ) {
     }
 
-    public function jsonSerialize(): array|\JsonSerializable
+    public function jsonSerialize(): array|\JsonSerializable|\stdClass
     {
-        return $this->data;
+        return $this->data === []
+            ? (object) []
+            : $this->data;
     }
 }
diff --git a/tests/Functional/Resources/Transactions/_fixtures/request/create_with_multiple_non_catalog_price_and_product.json b/tests/Functional/Resources/Transactions/_fixtures/request/create_with_multiple_non_catalog_price_and_product.json
index c652d39..8d744ff 100644
--- a/tests/Functional/Resources/Transactions/_fixtures/request/create_with_multiple_non_catalog_price_and_product.json
+++ b/tests/Functional/Resources/Transactions/_fixtures/request/create_with_multiple_non_catalog_price_and_product.json
@@ -6,7 +6,7 @@
                 "description": "Annual (per seat)",
                 "name": "Annual (per seat)",
                 "product": {
-                    "custom_data": [],
+                    "custom_data": {},
                     "description": "Some description",
                     "image_url": "https://paddle-sandbox.s3.amazonaws.com/user/10889/2nmP8MQSret0aWeDemRw_icon1.png",
                     "name": "Annual (per seat)",
diff --git a/tests/Functional/Resources/Transactions/_fixtures/request/create_with_non_catalog_price_and_product.json b/tests/Functional/Resources/Transactions/_fixtures/request/create_with_non_catalog_price_and_product.json
index e665ce2..3d1f134 100644
--- a/tests/Functional/Resources/Transactions/_fixtures/request/create_with_non_catalog_price_and_product.json
+++ b/tests/Functional/Resources/Transactions/_fixtures/request/create_with_non_catalog_price_and_product.json
@@ -6,7 +6,7 @@
                 "description": "Annual (per seat)",
                 "name": "Annual (per seat)",
                 "product": {
-                    "custom_data": [],
+                    "custom_data": {},
                     "description": "Some description",
                     "image_url": "https://paddle-sandbox.s3.amazonaws.com/user/10889/2nmP8MQSret0aWeDemRw_icon1.png",
                     "name": "Annual (per seat)",
diff --git a/tests/Functional/Resources/Transactions/_fixtures/request/preview_with_multiple_non_catalog_price_and_product.json b/tests/Functional/Resources/Transactions/_fixtures/request/preview_with_multiple_non_catalog_price_and_product.json
index d43c8ef..9af26ef 100644
--- a/tests/Functional/Resources/Transactions/_fixtures/request/preview_with_multiple_non_catalog_price_and_product.json
+++ b/tests/Functional/Resources/Transactions/_fixtures/request/preview_with_multiple_non_catalog_price_and_product.json
@@ -6,7 +6,7 @@
                 "description": "Annual (per seat)",
                 "name": "Annual (per seat)",
                 "product": {
-                    "custom_data": [],
+                    "custom_data": {},
                     "description": "Some description",
                     "image_url": "https://paddle-sandbox.s3.amazonaws.com/user/10889/2nmP8MQSret0aWeDemRw_icon1.png",
                     "name": "Annual (per seat)",
diff --git a/tests/Functional/Resources/Transactions/_fixtures/request/preview_with_non_catalog_price_and_product.json b/tests/Functional/Resources/Transactions/_fixtures/request/preview_with_non_catalog_price_and_product.json
index 1473e76..fb433a7 100644
--- a/tests/Functional/Resources/Transactions/_fixtures/request/preview_with_non_catalog_price_and_product.json
+++ b/tests/Functional/Resources/Transactions/_fixtures/request/preview_with_non_catalog_price_and_product.json
@@ -6,7 +6,7 @@
                 "description": "Annual (per seat)",
                 "name": "Annual (per seat)",
                 "product": {
-                    "custom_data": [],
+                    "custom_data": {},
                     "description": "Some description",
                     "image_url": "https://paddle-sandbox.s3.amazonaws.com/user/10889/2nmP8MQSret0aWeDemRw_icon1.png",
                     "name": "Annual (per seat)",
diff --git a/tests/Functional/Resources/Transactions/_fixtures/request/update_with_non_catalog_price_and_product.json b/tests/Functional/Resources/Transactions/_fixtures/request/update_with_non_catalog_price_and_product.json
index c652d39..8d744ff 100644
--- a/tests/Functional/Resources/Transactions/_fixtures/request/update_with_non_catalog_price_and_product.json
+++ b/tests/Functional/Resources/Transactions/_fixtures/request/update_with_non_catalog_price_and_product.json
@@ -6,7 +6,7 @@
                 "description": "Annual (per seat)",
                 "name": "Annual (per seat)",
                 "product": {
-                    "custom_data": [],
+                    "custom_data": {},
                     "description": "Some description",
                     "image_url": "https://paddle-sandbox.s3.amazonaws.com/user/10889/2nmP8MQSret0aWeDemRw_icon1.png",
                     "name": "Annual (per seat)",