diff --git a/tests/Models/NullBillElementTest.php b/tests/Models/NullBillElementTest.php
index 6e5eced..3f919ce 100755
--- a/tests/Models/NullBillElementTest.php
+++ b/tests/Models/NullBillElementTest.php
@@ -47,6 +47,6 @@ final class NullBillElementTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullBillElement(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }
diff --git a/tests/Models/NullBillTest.php b/tests/Models/NullBillTest.php
index fc6f7d7..c7031e9 100755
--- a/tests/Models/NullBillTest.php
+++ b/tests/Models/NullBillTest.php
@@ -47,6 +47,6 @@ final class NullBillTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullBill(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }
diff --git a/tests/Models/NullBillTypeTest.php b/tests/Models/NullBillTypeTest.php
index 0ed8740..9a76209 100755
--- a/tests/Models/NullBillTypeTest.php
+++ b/tests/Models/NullBillTypeTest.php
@@ -47,6 +47,6 @@ final class NullBillTypeTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullBillType(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }
diff --git a/tests/Models/NullSubscriptionTest.php b/tests/Models/NullSubscriptionTest.php
index 8ff15d9..c5efdf4 100644
--- a/tests/Models/NullSubscriptionTest.php
+++ b/tests/Models/NullSubscriptionTest.php
@@ -47,6 +47,6 @@ final class NullSubscriptionTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullSubscription(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }
diff --git a/tests/Models/Price/NullPriceTest.php b/tests/Models/Price/NullPriceTest.php
index dcd1609..e667445 100644
--- a/tests/Models/Price/NullPriceTest.php
+++ b/tests/Models/Price/NullPriceTest.php
@@ -47,6 +47,6 @@ final class NullPriceTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullPrice(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }
diff --git a/tests/Models/Tax/NullTaxCombinationTest.php b/tests/Models/Tax/NullTaxCombinationTest.php
index d93caec..bb033a2 100644
--- a/tests/Models/Tax/NullTaxCombinationTest.php
+++ b/tests/Models/Tax/NullTaxCombinationTest.php
@@ -47,6 +47,6 @@ final class NullTaxCombinationTest extends \PHPUnit\Framework\TestCase
     public function testJsonSerialize() : void
     {
         $null = new NullTaxCombination(2);
-        self::assertEquals(['id' => 2], $null);
+        self::assertEquals(['id' => 2], $null->jsonSerialize());
     }
 }