diff --git a/composer.json b/composer.json
index 74b1c20ad..ae36a252a 100644
--- a/composer.json
+++ b/composer.json
@@ -111,7 +111,7 @@
     "code:md": "phpmd src text codesize, naming, unusedcode",
     "code:md:gh": "phpmd src github codesize, naming, unusedcode",
     "code:style": "phpcs ./src/ --standard=PSR12 -n",
-    "code:fix": "php-cs-fixer fix ./src/ --rules=@PSR12,native_function_invocation --allow-risky=yes --show-progress=none",
+    "code:fix": "php-cs-fixer fix ./src/ --rules=@PSR12,native_function_invocation,nullable_type_declaration_for_default_null_value --allow-risky=yes --show-progress=none",
     "test": "@test:integration",
     "test:integration": "phpunit -c ./ --testsuite=IntegrationTests",
     "test:coverage": "phpunit -c ./ --testsuite=IntegrationTests --coverage-text --coverage-clover=build/logs/clover.xml",
diff --git a/src/Assets/Cache.php b/src/Assets/Cache.php
index 847c9fffe..7df8dbbd4 100644
--- a/src/Assets/Cache.php
+++ b/src/Assets/Cache.php
@@ -180,7 +180,7 @@ public function createKeyFromPath(string $path, string $relativePath): string
     /**
      * Creates key from an Asset source: "$filename_$ext_$tag__VERSION__MD5".
      */
-    public function createKeyFromAsset(Asset $asset, array $tags = null): string
+    public function createKeyFromAsset(Asset $asset, ?array $tags = null): string
     {
         $tags = implode('_', $tags ?? []);
 
diff --git a/src/Assets/Url.php b/src/Assets/Url.php
index 8bfa73459..c02030516 100644
--- a/src/Assets/Url.php
+++ b/src/Assets/Url.php
@@ -51,7 +51,7 @@ class Url
      * @param Page|Asset|string|null $value
      * @param array|null             $options Rendering options, e.g.: ['canonical' => true, 'format' => 'html', 'language' => 'fr']
      */
-    public function __construct(Builder $builder, $value, array $options = null)
+    public function __construct(Builder $builder, $value, ?array $options = null)
     {
         $this->builder = $builder;
         $this->config = $builder->getConfig();
diff --git a/src/Builder.php b/src/Builder.php
index c86e6c84b..5e62395e4 100644
--- a/src/Builder.php
+++ b/src/Builder.php
@@ -98,7 +98,7 @@ class Builder implements LoggerAwareInterface
      * @param Config|array|null    $config
      * @param LoggerInterface|null $logger
      */
-    public function __construct($config = null, LoggerInterface $logger = null)
+    public function __construct($config = null, ?LoggerInterface $logger = null)
     {
         // set logger
         if ($logger === null) {
@@ -202,7 +202,7 @@ public function getConfig(): Config
     /**
      * Config::setSourceDir() alias.
      */
-    public function setSourceDir(string $sourceDir = null): self
+    public function setSourceDir(?string $sourceDir = null): self
     {
         $this->config->setSourceDir($sourceDir);
 
@@ -212,7 +212,7 @@ public function setSourceDir(string $sourceDir = null): self
     /**
      * Config::setDestinationDir() alias.
      */
-    public function setDestinationDir(string $destinationDir = null): self
+    public function setDestinationDir(?string $destinationDir = null): self
     {
         $this->config->setDestinationDir($destinationDir);
 
@@ -278,7 +278,7 @@ public function setData(array $data): void
     /**
      * Returns data collection.
      */
-    public function getData(string $language = null): array
+    public function getData(?string $language = null): array
     {
         if ($language) {
             if (empty($this->data[$language])) {
diff --git a/src/Collection/Collection.php b/src/Collection/Collection.php
index 11990a190..e93aa708d 100644
--- a/src/Collection/Collection.php
+++ b/src/Collection/Collection.php
@@ -217,7 +217,7 @@ public function getIterator(): \ArrayIterator
     /**
      * {@inheritdoc}
      */
-    public function usort(\Closure $callback = null): CollectionInterface
+    public function usort(?\Closure $callback = null): CollectionInterface
     {
         $callback ? usort($this->items, $callback) : usort($this->items, function ($a, $b) {
             if ($a == $b) {
diff --git a/src/Collection/CollectionInterface.php b/src/Collection/CollectionInterface.php
index 915f09e8c..2af9fcd1f 100644
--- a/src/Collection/CollectionInterface.php
+++ b/src/Collection/CollectionInterface.php
@@ -93,7 +93,7 @@ public function getIterator(): \ArrayIterator;
      *
      * @param \Closure|null $callback
      */
-    public function usort(\Closure $callback = null): self;
+    public function usort(?\Closure $callback = null): self;
 
     /**
      * Reverse items.
diff --git a/src/Collection/Menu/Entry.php b/src/Collection/Menu/Entry.php
index 5f6c26c53..1c3a4a805 100644
--- a/src/Collection/Menu/Entry.php
+++ b/src/Collection/Menu/Entry.php
@@ -41,7 +41,7 @@ public function getName(): ?string
     /**
      * Set the menu entry URL.
      */
-    public function setUrl(string $value = null): self
+    public function setUrl(?string $value = null): self
     {
         $this->offsetSet('url', $value);
 
diff --git a/src/Collection/Page/Collection.php b/src/Collection/Page/Collection.php
index ddd969879..0f4a910a4 100644
--- a/src/Collection/Page/Collection.php
+++ b/src/Collection/Page/Collection.php
@@ -160,7 +160,7 @@ public function filter(\Closure $callback): self
     /**
      * {@inheritdoc}
      */
-    public function usort(\Closure $callback = null): self
+    public function usort(?\Closure $callback = null): self
     {
         return parent::usort($callback);
     }
diff --git a/src/Config.php b/src/Config.php
index 373b9fcef..8149329cb 100644
--- a/src/Config.php
+++ b/src/Config.php
@@ -176,7 +176,7 @@ public function get(string $key, ?string $language = null, bool $fallback = true
      *
      * @throws \InvalidArgumentException
      */
-    public function setSourceDir(string $sourceDir = null): self
+    public function setSourceDir(?string $sourceDir = null): self
     {
         if ($sourceDir === null) {
             $sourceDir = getcwd();
@@ -202,7 +202,7 @@ public function getSourceDir(): string
      *
      * @throws \InvalidArgumentException
      */
-    public function setDestinationDir(string $destinationDir = null): self
+    public function setDestinationDir(?string $destinationDir = null): self
     {
         if ($destinationDir === null) {
             $destinationDir = $this->sourceDir;
diff --git a/src/Exception/ConfigException.php b/src/Exception/ConfigException.php
index d9475ab5c..7f5b95dfa 100644
--- a/src/Exception/ConfigException.php
+++ b/src/Exception/ConfigException.php
@@ -15,7 +15,7 @@
 
 class ConfigException extends \RuntimeException implements ExceptionInterface
 {
-    public function __construct(string $message, int $code = 0, \Throwable $previous = null)
+    public function __construct(string $message, int $code = 0, ?\Throwable $previous = null)
     {
         parent::__construct("Configuration: $message", $code, $previous);
     }
diff --git a/src/Exception/RuntimeException.php b/src/Exception/RuntimeException.php
index aa5a7b1b2..da97a8c5d 100644
--- a/src/Exception/RuntimeException.php
+++ b/src/Exception/RuntimeException.php
@@ -19,7 +19,7 @@ class RuntimeException extends \RuntimeException implements ExceptionInterface
     private $pageLine;
     private $pageCol;
 
-    public function __construct(string $message, string $pageFile = null, int $pageLine = null, int $pageCol = null, \Throwable $previous = null)
+    public function __construct(string $message, ?string $pageFile = null, ?int $pageLine = null, ?int $pageCol = null, ?\Throwable $previous = null)
     {
         $this->pageFile = $pageFile;
         $this->pageLine = $pageLine;
diff --git a/src/Logger/PrintLogger.php b/src/Logger/PrintLogger.php
index 45470e6bd..0ea7e35ad 100644
--- a/src/Logger/PrintLogger.php
+++ b/src/Logger/PrintLogger.php
@@ -38,7 +38,7 @@ class PrintLogger extends AbstractLogger
     /**
      * Print only the $printLevelMax.
      */
-    public function __construct(int $printLevelMax = null)
+    public function __construct(?int $printLevelMax = null)
     {
         $this->printLevelMax = $printLevelMax;
     }
diff --git a/src/Renderer/Extension/Core.php b/src/Renderer/Extension/Core.php
index 331a4a807..5fbad8412 100644
--- a/src/Renderer/Extension/Core.php
+++ b/src/Renderer/Extension/Core.php
@@ -265,7 +265,7 @@ public function sortByDate(\Traversable $collection, string $variable = 'date',
      * @param Page|Asset|string|null $value
      * @param array|null             $options
      */
-    public function url(array $context, $value = null, array $options = null): string
+    public function url(array $context, $value = null, ?array $options = null): string
     {
         $optionsLang = [];
         $optionsLang['language'] = (string) $context['site']['language'];
diff --git a/src/Renderer/Language.php b/src/Renderer/Language.php
index dcfa1bd3e..b506ad5df 100644
--- a/src/Renderer/Language.php
+++ b/src/Renderer/Language.php
@@ -26,7 +26,7 @@ class Language
     /** @var string Current language. */
     protected $language;
 
-    public function __construct(\Cecil\Config $config, string $language = null)
+    public function __construct(\Cecil\Config $config, ?string $language = null)
     {
         $this->config = $config;
         $this->language = $language;
diff --git a/src/Renderer/Site.php b/src/Renderer/Site.php
index aa56b9665..1d7ca8780 100644
--- a/src/Renderer/Site.php
+++ b/src/Renderer/Site.php
@@ -121,7 +121,7 @@ public function offsetUnset($offset): void
      *
      * @throws \DomainException
      */
-    public function getPage(string $id, string $language = null): ?CollectionPage
+    public function getPage(string $id, ?string $language = null): ?CollectionPage
     {
         $pageId = $id;
         $language = $language ?? $this->language;
diff --git a/src/Renderer/Twig.php b/src/Renderer/Twig.php
index f5aed51e8..5431894f7 100644
--- a/src/Renderer/Twig.php
+++ b/src/Renderer/Twig.php
@@ -115,7 +115,7 @@ public function __construct(Builder $builder, $templatesPath)
         $this->twig->registerUndefinedFilterCallback(function ($name) {
             switch ($name) {
                 case 'localizeddate':
-                    return new \Twig\TwigFilter($name, function (\DateTime $value = null) {
+                    return new \Twig\TwigFilter($name, function (?\DateTime $value = null) {
                         return date($this->builder->getConfig()->get('date.format') ?? 'F j, Y', $value->getTimestamp());
                     });
             }
diff --git a/src/Step/Optimize/AbstractOptimize.php b/src/Step/Optimize/AbstractOptimize.php
index 1a0af045d..b6eac2666 100644
--- a/src/Step/Optimize/AbstractOptimize.php
+++ b/src/Step/Optimize/AbstractOptimize.php
@@ -122,7 +122,7 @@ abstract public function processFile(\Symfony\Component\Finder\SplFileInfo $file
     /**
      * Encode file content.
      */
-    public function encode(string $content = null): ?string
+    public function encode(?string $content = null): ?string
     {
         return $content;
     }
@@ -130,7 +130,7 @@ public function encode(string $content = null): ?string
     /**
      * Decode file content.
      */
-    public function decode(string $content = null): ?string
+    public function decode(?string $content = null): ?string
     {
         return $content;
     }
diff --git a/src/Step/Optimize/Html.php b/src/Step/Optimize/Html.php
index 9440b91c3..d21918c22 100644
--- a/src/Step/Optimize/Html.php
+++ b/src/Step/Optimize/Html.php
@@ -65,7 +65,7 @@ public function processFile(\Symfony\Component\Finder\SplFileInfo $file): string
     /**
      * {@inheritdoc}
      */
-    public function encode(string $content = null): ?string
+    public function encode(?string $content = null): ?string
     {
         return json_encode($content);
     }
@@ -73,7 +73,7 @@ public function encode(string $content = null): ?string
     /**
      * {@inheritdoc}
      */
-    public function decode(string $content = null): ?string
+    public function decode(?string $content = null): ?string
     {
         return json_decode((string) $content);
     }
diff --git a/src/Step/Optimize/Images.php b/src/Step/Optimize/Images.php
index 1cf9453b6..c54446005 100644
--- a/src/Step/Optimize/Images.php
+++ b/src/Step/Optimize/Images.php
@@ -62,7 +62,7 @@ public function processFile(\Symfony\Component\Finder\SplFileInfo $file): string
     /**
      * {@inheritdoc}
      */
-    public function encode(string $content = null): ?string
+    public function encode(?string $content = null): ?string
     {
         return base64_encode((string) $content);
     }
@@ -70,7 +70,7 @@ public function encode(string $content = null): ?string
     /**
      * {@inheritdoc}
      */
-    public function decode(string $content = null): ?string
+    public function decode(?string $content = null): ?string
     {
         return base64_decode((string) $content);
     }
diff --git a/src/Step/StaticFiles/Copy.php b/src/Step/StaticFiles/Copy.php
index d59ac03b3..f35080772 100644
--- a/src/Step/StaticFiles/Copy.php
+++ b/src/Step/StaticFiles/Copy.php
@@ -100,7 +100,7 @@ public function process(): void
     /**
      * Copying (mirror) files.
      */
-    protected function copy(string $from, string $to = null, array $exclude = null): bool
+    protected function copy(string $from, ?string $to = null, ?array $exclude = null): bool
     {
         if (Util\File::getFS()->exists($from)) {
             $finder = Finder::create()