diff --git a/tests/Models/NullInvestmentTest.php b/tests/Models/NullInvestmentTest.php
index 9c85b5e..6479ee7 100644
--- a/tests/Models/NullInvestmentTest.php
+++ b/tests/Models/NullInvestmentTest.php
@@ -47,6 +47,6 @@ final class NullInvestmentTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullInvestment(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }