diff --git a/src/Resources/AbstractUnzerResource.php b/src/Resources/AbstractUnzerResource.php
index 7d9e3ada..59a3750d 100644
--- a/src/Resources/AbstractUnzerResource.php
+++ b/src/Resources/AbstractUnzerResource.php
@@ -9,6 +9,7 @@
 namespace UnzerSDK\Resources;
 
 use DateTime;
+use ReflectionClass;
 use ReflectionException;
 use ReflectionProperty;
 use RuntimeException;
@@ -495,10 +496,21 @@ public function getExternalId(): ?string
      */
     private function exposeProperties(): array
     {
-        $properties = get_object_vars($this);
-        foreach ($properties as $property => $value) {
-            if (self::propertyShouldBeSkipped($property, $value)) {
-                unset($properties[$property]);
+        $reflectionClass = new ReflectionClass(static::class);
+        $reflectionProperties = $reflectionClass->getProperties(ReflectionProperty::IS_PROTECTED); // only send protected properties
+
+        $properties = [];
+        foreach ($reflectionProperties as $propertyObject) {
+            $property = $propertyObject->getName();
+            $value = $propertyObject->getValue($this);
+
+            // do not send properties that are set to null
+            if ($value === null) {
+                continue;
+            }
+
+            // do not send id property if it is empty
+            if ($property === 'id' && empty($value)) {
                 continue;
             }
 
@@ -516,7 +528,6 @@ private function exposeProperties(): array
             // handle additional values
             if ($property === 'additionalAttributes') {
                 if (!is_array($value) || empty($value)) {
-                    unset($properties[$property]);
                     continue;
                 }
                 $value = $this->exposeAdditionalAttributes($value);
diff --git a/test/unit/Resources/AbstractUnzerResourceTest.php b/test/unit/Resources/AbstractUnzerResourceTest.php
index 2212a5c1..5e6b0194 100644
--- a/test/unit/Resources/AbstractUnzerResourceTest.php
+++ b/test/unit/Resources/AbstractUnzerResourceTest.php
@@ -414,6 +414,19 @@ public function additionalAttributesShouldBeSettable(): void
         $this->assertEquals(1234.567, $paypage->expose()['additionalAttributes']['effectiveInterestRate']);
     }
 
+    /**
+     * @test
+     */
+    public function parentPrivatePropertiesShouldNotCauseAnException(): void
+    {
+        $metadata = new Metadata();
+        $metadata->setParentResource(new Unzer('s-priv-123'));
+        $metadata->setSpecialParams(['something' => 'special']);
+
+        $result = $metadata->jsonSerialize();
+        $this->assertEquals('{"something":"special"}', $result);
+    }
+
     //<editor-fold desc="Data Providers">
 
     /**