diff --git a/tests/Models/NullCostCenterTest.php b/tests/Models/NullCostCenterTest.php
index 9566399..d3a0e77 100755
--- a/tests/Models/NullCostCenterTest.php
+++ b/tests/Models/NullCostCenterTest.php
@@ -47,6 +47,6 @@ final class NullCostCenterTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullCostCenter(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }
diff --git a/tests/Models/NullCostObjectTest.php b/tests/Models/NullCostObjectTest.php
index 499fb0b..11d7819 100755
--- a/tests/Models/NullCostObjectTest.php
+++ b/tests/Models/NullCostObjectTest.php
@@ -47,6 +47,6 @@ final class NullCostObjectTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullCostObject(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }