diff --git a/Models/NullItem.php b/Models/NullItem.php
index e762511..4d569f7 100755
--- a/Models/NullItem.php
+++ b/Models/NullItem.php
@@ -36,4 +36,12 @@ final class NullItem extends Item
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullItemAttribute.php b/Models/NullItemAttribute.php
index 035ef03..f44ae50 100755
--- a/Models/NullItemAttribute.php
+++ b/Models/NullItemAttribute.php
@@ -36,4 +36,12 @@ final class NullItemAttribute extends ItemAttribute
         parent::__construct();
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullItemAttributeType.php b/Models/NullItemAttributeType.php
index e320798..c9754a6 100755
--- a/Models/NullItemAttributeType.php
+++ b/Models/NullItemAttributeType.php
@@ -35,4 +35,12 @@ final class NullItemAttributeType extends ItemAttributeType
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullItemAttributeTypeL11n.php b/Models/NullItemAttributeTypeL11n.php
index 10d39be..7ea9d59 100755
--- a/Models/NullItemAttributeTypeL11n.php
+++ b/Models/NullItemAttributeTypeL11n.php
@@ -36,4 +36,12 @@ final class NullItemAttributeTypeL11n extends ItemAttributeTypeL11n
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullItemAttributeValue.php b/Models/NullItemAttributeValue.php
index 172ea65..a424fe8 100755
--- a/Models/NullItemAttributeValue.php
+++ b/Models/NullItemAttributeValue.php
@@ -35,4 +35,12 @@ final class NullItemAttributeValue extends ItemAttributeValue
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullItemL11n.php b/Models/NullItemL11n.php
index a552c55..22dd8cd 100755
--- a/Models/NullItemL11n.php
+++ b/Models/NullItemL11n.php
@@ -36,4 +36,12 @@ final class NullItemL11n extends ItemL11n
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullItemL11nType.php b/Models/NullItemL11nType.php
index a6353b3..bde0323 100755
--- a/Models/NullItemL11nType.php
+++ b/Models/NullItemL11nType.php
@@ -35,4 +35,12 @@ final class NullItemL11nType extends ItemL11nType
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }