diff --git a/tests/Models/NullCostCenterTest.php b/tests/Models/NullCostCenterTest.php
index bcf43e0..9566399 100755
--- a/tests/Models/NullCostCenterTest.php
+++ b/tests/Models/NullCostCenterTest.php
@@ -23,7 +23,7 @@ final class NullCostCenterTest extends \PHPUnit\Framework\TestCase
 {
     /**
      * @covers Modules\Accounting\Models\NullCostCenter
-     * @group framework
+     * @group module
      */
     public function testNull() : void
     {
@@ -32,11 +32,21 @@ final class NullCostCenterTest extends \PHPUnit\Framework\TestCase
 
     /**
      * @covers Modules\Accounting\Models\NullCostCenter
-     * @group framework
+     * @group module
      */
     public function testId() : void
     {
         $null = new NullCostCenter(2);
         self::assertEquals(2, $null->id);
     }
+
+    /**
+     * @covers Modules\Accounting\Models\NullCostCenter
+     * @group module
+     */
+    public function testJsonSerialize() : void
+    {
+        $null = new NullCostCenter(2);
+        self::assertEquals(['id' => 2], $null);
+    }
 }
diff --git a/tests/Models/NullCostObjectTest.php b/tests/Models/NullCostObjectTest.php
index e4f5ead..499fb0b 100755
--- a/tests/Models/NullCostObjectTest.php
+++ b/tests/Models/NullCostObjectTest.php
@@ -23,7 +23,7 @@ final class NullCostObjectTest extends \PHPUnit\Framework\TestCase
 {
     /**
      * @covers Modules\Accounting\Models\NullCostObject
-     * @group framework
+     * @group module
      */
     public function testNull() : void
     {
@@ -32,11 +32,21 @@ final class NullCostObjectTest extends \PHPUnit\Framework\TestCase
 
     /**
      * @covers Modules\Accounting\Models\NullCostObject
-     * @group framework
+     * @group module
      */
     public function testId() : void
     {
         $null = new NullCostObject(2);
         self::assertEquals(2, $null->id);
     }
+
+    /**
+     * @covers Modules\Accounting\Models\NullCostObject
+     * @group module
+     */
+    public function testJsonSerialize() : void
+    {
+        $null = new NullCostObject(2);
+        self::assertEquals(['id' => 2], $null);
+    }
 }