diff --git a/tests/Models/NullBillElementTest.php b/tests/Models/NullBillElementTest.php
index 9db5122..6f52e97 100755
--- a/tests/Models/NullBillElementTest.php
+++ b/tests/Models/NullBillElementTest.php
@@ -39,4 +39,14 @@ final class NullBillElementTest extends \PHPUnit\Framework\TestCase
         $null = new NullBillElement(2);
         self::assertEquals(2, $null->id);
     }
+
+    /**
+     * @covers Modules\Billing\Models\NullBillElement
+     * @group module
+     */
+    public function testJsonSerialize() : void
+    {
+        $null = new NullBillElement(2);
+        self::assertEquals(['id' => 2], $null);
+    }
 }
diff --git a/tests/Models/NullBillTest.php b/tests/Models/NullBillTest.php
index 7829489..8a3de0f 100755
--- a/tests/Models/NullBillTest.php
+++ b/tests/Models/NullBillTest.php
@@ -39,4 +39,14 @@ final class NullBillTest extends \PHPUnit\Framework\TestCase
         $null = new NullBill(2);
         self::assertEquals(2, $null->id);
     }
+
+    /**
+     * @covers Modules\Billing\Models\NullBill
+     * @group module
+     */
+    public function testJsonSerialize() : void
+    {
+        $null = new NullBill(2);
+        self::assertEquals(['id' => 2], $null);
+    }
 }
diff --git a/tests/Models/NullBillTypeTest.php b/tests/Models/NullBillTypeTest.php
index 4e04f20..cdfdddf 100755
--- a/tests/Models/NullBillTypeTest.php
+++ b/tests/Models/NullBillTypeTest.php
@@ -39,4 +39,14 @@ final class NullBillTypeTest extends \PHPUnit\Framework\TestCase
         $null = new NullBillType(2);
         self::assertEquals(2, $null->id);
     }
+
+    /**
+     * @covers Modules\Billing\Models\NullBillType
+     * @group module
+     */
+    public function testJsonSerialize() : void
+    {
+        $null = new NullBillType(2);
+        self::assertEquals(['id' => 2], $null);
+    }
 }
diff --git a/tests/Models/NullSubscriptionTest.php b/tests/Models/NullSubscriptionTest.php
new file mode 100644
index 0000000..dd6a430
--- /dev/null
+++ b/tests/Models/NullSubscriptionTest.php
@@ -0,0 +1,52 @@
+<?php
+/**
+ * Jingga
+ *
+ * PHP Version 8.1
+ *
+ * @package   tests
+ * @copyright Dennis Eichhorn
+ * @license   OMS License 2.0
+ * @version   1.0.0
+ * @link      https://jingga.app
+ */
+declare(strict_types=1);
+
+namespace Modules\Billing\tests\Models;
+
+use Modules\Billing\Models\NullSubscription;
+
+/**
+ * @internal
+ */
+final class NullSubscriptionTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @covers Modules\Billing\Models\NullSubscription
+     * @group framework
+     */
+    public function testNull() : void
+    {
+        self::assertInstanceOf('\Modules\Billing\Models\Subscription', new NullSubscription());
+    }
+
+    /**
+     * @covers Modules\Billing\Models\NullSubscription
+     * @group framework
+     */
+    public function testId() : void
+    {
+        $null = new NullSubscription(2);
+        self::assertEquals(2, $null->id);
+    }
+
+    /**
+     * @covers Modules\Billing\Models\NullSubscription
+     * @group module
+     */
+    public function testJsonSerialize() : void
+    {
+        $null = new NullSubscription(2);
+        self::assertEquals(['id' => 2], $null);
+    }
+}
diff --git a/tests/Models/Price/NullPriceTest.php b/tests/Models/Price/NullPriceTest.php
new file mode 100644
index 0000000..dcd1609
--- /dev/null
+++ b/tests/Models/Price/NullPriceTest.php
@@ -0,0 +1,52 @@
+<?php
+/**
+ * Jingga
+ *
+ * PHP Version 8.1
+ *
+ * @package   tests
+ * @copyright Dennis Eichhorn
+ * @license   OMS License 2.0
+ * @version   1.0.0
+ * @link      https://jingga.app
+ */
+declare(strict_types=1);
+
+namespace Modules\Billing\tests\Models\Price;
+
+use Modules\Billing\Models\Price\NullPrice;
+
+/**
+ * @internal
+ */
+final class NullPriceTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @covers Modules\Billing\Models\Price\NullPrice
+     * @group module
+     */
+    public function testNull() : void
+    {
+        self::assertInstanceOf('\Modules\Billing\Models\Price\Price', new NullPrice());
+    }
+
+    /**
+     * @covers Modules\Billing\Models\Price\NullPrice
+     * @group module
+     */
+    public function testId() : void
+    {
+        $null = new NullPrice(2);
+        self::assertEquals(2, $null->id);
+    }
+
+    /**
+     * @covers Modules\Billing\Models\Price\NullPrice
+     * @group module
+     */
+    public function testJsonSerialize() : void
+    {
+        $null = new NullPrice(2);
+        self::assertEquals(['id' => 2], $null);
+    }
+}
diff --git a/tests/Models/Tax/NullTaxCombinationTest.php b/tests/Models/Tax/NullTaxCombinationTest.php
new file mode 100644
index 0000000..d93caec
--- /dev/null
+++ b/tests/Models/Tax/NullTaxCombinationTest.php
@@ -0,0 +1,52 @@
+<?php
+/**
+ * Jingga
+ *
+ * PHP Version 8.1
+ *
+ * @package   tests
+ * @copyright Dennis Eichhorn
+ * @license   OMS License 2.0
+ * @version   1.0.0
+ * @link      https://jingga.app
+ */
+declare(strict_types=1);
+
+namespace Modules\Billing\tests\Models\Tax;
+
+use Modules\Billing\Models\Tax\NullTaxCombination;
+
+/**
+ * @internal
+ */
+final class NullTaxCombinationTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @covers Modules\Billing\Models\Tax\NullTaxCombination
+     * @group module
+     */
+    public function testNull() : void
+    {
+        self::assertInstanceOf('\Modules\Billing\Models\Tax\TaxCombination', new NullTaxCombination());
+    }
+
+    /**
+     * @covers Modules\Billing\Models\Tax\NullTaxCombination
+     * @group module
+     */
+    public function testId() : void
+    {
+        $null = new NullTaxCombination(2);
+        self::assertEquals(2, $null->id);
+    }
+
+    /**
+     * @covers Modules\Billing\Models\Tax\NullTaxCombination
+     * @group module
+     */
+    public function testJsonSerialize() : void
+    {
+        $null = new NullTaxCombination(2);
+        self::assertEquals(['id' => 2], $null);
+    }
+}