diff --git a/tests/Models/NullAttributeTest.php b/tests/Models/NullAttributeTest.php
index ed3748d..b7404fd 100644
--- a/tests/Models/NullAttributeTest.php
+++ b/tests/Models/NullAttributeTest.php
@@ -47,6 +47,6 @@ final class NullAttributeTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullAttribute(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }
diff --git a/tests/Models/NullAttributeTypeTest.php b/tests/Models/NullAttributeTypeTest.php
index 173221b..379a215 100644
--- a/tests/Models/NullAttributeTypeTest.php
+++ b/tests/Models/NullAttributeTypeTest.php
@@ -47,6 +47,6 @@ final class NullAttributeTypeTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullAttributeType(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }
diff --git a/tests/Models/NullAttributeValueTest.php b/tests/Models/NullAttributeValueTest.php
index 34c1411..6cbe2ae 100644
--- a/tests/Models/NullAttributeValueTest.php
+++ b/tests/Models/NullAttributeValueTest.php
@@ -47,6 +47,6 @@ final class NullAttributeValueTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullAttributeValue(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }