diff --git a/src/Resources/Product.php b/src/Resources/Product.php
index da4f34e..c5e4ef6 100644
--- a/src/Resources/Product.php
+++ b/src/Resources/Product.php
@@ -7,6 +7,7 @@
 use MorningTrain\Economic\Attributes\Resources\GetCollection;
 use MorningTrain\Economic\Attributes\Resources\GetSingle;
 use MorningTrain\Economic\Attributes\Resources\Properties\Filterable;
+use MorningTrain\Economic\Attributes\Resources\Properties\PrimaryKey;
 use MorningTrain\Economic\Attributes\Resources\Properties\ResourceType;
 use MorningTrain\Economic\Attributes\Resources\Properties\Sortable;
 use MorningTrain\Economic\Classes\EconomicCollection;
@@ -73,6 +74,9 @@ class Product extends Resource
     #[Sortable]
     public string $year;
 
+    #[PrimaryKey]
+    public int $productNumber;
+
     public static function create(DateTime $fromDate, DateTime $toDate): static
     {
         return static::createRequest(compact('fromDate', 'toDate'));
diff --git a/tests/Unit/InvoiceTest.php b/tests/Unit/InvoiceTest.php
index ed25bd5..82bcf60 100644
--- a/tests/Unit/InvoiceTest.php
+++ b/tests/Unit/InvoiceTest.php
@@ -31,6 +31,9 @@
                     ],
                     'lines' => [
                         [
+                            'product' => [
+                                'productNumber' => 1,
+                            ],
                             'quantity' => 1,
                             'unitNetPrice' => 500,
                         ],