diff --git a/Form/Type/SiteType.php b/Form/Type/SiteType.php
index 586bc98..f9ae88a 100644
--- a/Form/Type/SiteType.php
+++ b/Form/Type/SiteType.php
@@ -26,7 +26,7 @@ public function configureOptions(OptionsResolver $resolver): void
     {
         $resolver->setDefaults([
             'choices' => $this->siteRepository->findAll(),
-            'choice_label' => static function (SiteInterface $site = null) {
+            'choice_label' => static function (?SiteInterface $site = null) {
                 return (string) $site;
             },
             'choice_value' => 'id',
diff --git a/Router/HostnameIdentifiedLoader.php b/Router/HostnameIdentifiedLoader.php
index f7791b2..8003e13 100644
--- a/Router/HostnameIdentifiedLoader.php
+++ b/Router/HostnameIdentifiedLoader.php
@@ -30,7 +30,7 @@ public function __construct(IdentifierMappingInterface $identifierMapping)
         $this->identifierMapping = $identifierMapping;
     }
 
-    public function load($resource, string $type = null): RouteCollection
+    public function load($resource, ?string $type = null): RouteCollection
     {
         $this->resourceStack[] = $resource;
 
@@ -52,7 +52,7 @@ public function load($resource, string $type = null): RouteCollection
         return $collection;
     }
 
-    public function supports($resource, string $type = null): bool
+    public function supports($resource, ?string $type = null): bool
     {
         return null === $type && !\in_array($resource, $this->resourceStack, true);
     }
diff --git a/Router/PathIdentifiedUrlGenerator.php b/Router/PathIdentifiedUrlGenerator.php
index 899060c..2411239 100644
--- a/Router/PathIdentifiedUrlGenerator.php
+++ b/Router/PathIdentifiedUrlGenerator.php
@@ -18,7 +18,7 @@ final class PathIdentifiedUrlGenerator implements UrlGeneratorInterface
     private $defaultIdentifier;
     private $urlGenerator;
 
-    public function __construct(UrlGeneratorInterface $urlGenerator, string $routeParameter = 'site', string $defaultIdentifier = null)
+    public function __construct(UrlGeneratorInterface $urlGenerator, string $routeParameter = 'site', ?string $defaultIdentifier = null)
     {
         $this->routeParameter = $routeParameter;
         $this->defaultIdentifier = $defaultIdentifier;
diff --git a/Site/IdentifierMapping.php b/Site/IdentifierMapping.php
index 7b1be03..7798ac3 100644
--- a/Site/IdentifierMapping.php
+++ b/Site/IdentifierMapping.php
@@ -39,7 +39,7 @@ public function findIdentifierByHostname(string $hostname): ?string
         return null;
     }
 
-    public function findHostnamesByIdentifier(string $identifier, string $locale = null): array
+    public function findHostnamesByIdentifier(string $identifier, ?string $locale = null): array
     {
         if (!isset($this->mapping[$identifier])) {
             return [];
diff --git a/Site/IdentifierMappingInterface.php b/Site/IdentifierMappingInterface.php
index eba8ad6..4be91c4 100644
--- a/Site/IdentifierMappingInterface.php
+++ b/Site/IdentifierMappingInterface.php
@@ -11,5 +11,5 @@ public function findIdentifierByHostname(string $hostname): ?string;
     /**
      * @return string[]
      */
-    public function findHostnamesByIdentifier(string $identifier, string $locale = null): array;
+    public function findHostnamesByIdentifier(string $identifier, ?string $locale = null): array;
 }
diff --git a/Site/PrefixedPathIdentifiedSiteResolver.php b/Site/PrefixedPathIdentifiedSiteResolver.php
index d4bb518..2f2c98c 100644
--- a/Site/PrefixedPathIdentifiedSiteResolver.php
+++ b/Site/PrefixedPathIdentifiedSiteResolver.php
@@ -17,9 +17,8 @@ final class PrefixedPathIdentifiedSiteResolver implements SiteResolverInterface
 
     /**
      * @param string[] $identifiers
-     * @param string   $defaultIdentifier
      */
-    public function __construct(SiteRepositoryInterface $siteRepository, array $identifiers, string $defaultIdentifier = null)
+    public function __construct(SiteRepositoryInterface $siteRepository, array $identifiers, ?string $defaultIdentifier = null)
     {
         $this->siteRepository = $siteRepository;
         $this->defaultIdentifier = $defaultIdentifier;
diff --git a/Tests/Site/IdentifierMappingTest.php b/Tests/Site/IdentifierMappingTest.php
index 4b1b306..2568e45 100644
--- a/Tests/Site/IdentifierMappingTest.php
+++ b/Tests/Site/IdentifierMappingTest.php
@@ -47,7 +47,7 @@ protected function setUp(): void
     /**
      * @dataProvider identifierMappingData
      */
-    public function testFindIdentifierByHostnameReturnsCorrectIdentifier(string $hostname, string $expectedIdentifier = null): void
+    public function testFindIdentifierByHostnameReturnsCorrectIdentifier(string $hostname, ?string $expectedIdentifier = null): void
     {
         $identifier = $this->identifierMapping->findIdentifierByHostname($hostname);