diff --git a/src/ProblemDetailsResponseFactory.php b/src/ProblemDetailsResponseFactory.php
index 1a940ae..5e193c6 100644
--- a/src/ProblemDetailsResponseFactory.php
+++ b/src/ProblemDetailsResponseFactory.php
@@ -220,7 +220,7 @@ public function __construct(
         }
         // Ensures type safety of the composed factory
         $this->responseFactory = $responseFactory;
-        if (! $jsonFlags) {
+        if ($jsonFlags === null) {
             $jsonFlags = JSON_UNESCAPED_SLASHES
                 | JSON_UNESCAPED_UNICODE
                 | JSON_PRESERVE_ZERO_FRACTION
diff --git a/test/ProblemDetailsResponseFactoryTest.php b/test/ProblemDetailsResponseFactoryTest.php
index 552462a..ac7ad67 100644
--- a/test/ProblemDetailsResponseFactoryTest.php
+++ b/test/ProblemDetailsResponseFactoryTest.php
@@ -144,7 +144,7 @@ public function testCreateResponseRemovesInvalidCharactersFromXmlKeys(string $he
             ],
         ];
 
-        if (stripos($expectedType, 'xml')) {
+        if (stripos($expectedType, 'xml') !== false) {
             $expectedKeyNames = [
                 'A_-',
                 '_A-',