diff --git a/tests/Models/NullExpenseElementTest.php b/tests/Models/NullExpenseElementTest.php
index 7ad83aa..c632762 100644
--- a/tests/Models/NullExpenseElementTest.php
+++ b/tests/Models/NullExpenseElementTest.php
@@ -47,6 +47,6 @@ final class NullExpenseElementTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullExpenseElement(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }
diff --git a/tests/Models/NullExpenseTest.php b/tests/Models/NullExpenseTest.php
index 92a6fc2..b7a68ee 100644
--- a/tests/Models/NullExpenseTest.php
+++ b/tests/Models/NullExpenseTest.php
@@ -47,6 +47,6 @@ final class NullExpenseTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullExpense(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }