diff --git a/Models/NullStock.php b/Models/NullStock.php
index b22c200..6eda25d 100755
--- a/Models/NullStock.php
+++ b/Models/NullStock.php
@@ -36,4 +36,12 @@ final class NullStock extends Stock
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullStockLocation.php b/Models/NullStockLocation.php
index 755eea4..bbbb650 100755
--- a/Models/NullStockLocation.php
+++ b/Models/NullStockLocation.php
@@ -36,4 +36,12 @@ final class NullStockLocation extends StockLocation
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullStockShelf.php b/Models/NullStockShelf.php
index 8f2bc32..37bd350 100755
--- a/Models/NullStockShelf.php
+++ b/Models/NullStockShelf.php
@@ -36,4 +36,12 @@ final class NullStockShelf extends StockShelf
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }