diff --git a/src/Cache.php b/src/Cache.php
index 6ecca1590..f861e4d96 100644
--- a/src/Cache.php
+++ b/src/Cache.php
@@ -30,7 +30,7 @@ class Cache extends BaseCache
     private $downloader;
     private $io;
 
-    public function setSymfonyRequire(string $symfonyRequire, RootPackageInterface $rootPackage, Downloader $downloader, IOInterface $io = null)
+    public function setSymfonyRequire(string $symfonyRequire, RootPackageInterface $rootPackage, Downloader $downloader, ?IOInterface $io = null)
     {
         $this->versionParser = new VersionParser();
         $this->symfonyRequire = $symfonyRequire;
diff --git a/src/Command/UpdateRecipesCommand.php b/src/Command/UpdateRecipesCommand.php
index c75507eca..15296efc1 100644
--- a/src/Command/UpdateRecipesCommand.php
+++ b/src/Command/UpdateRecipesCommand.php
@@ -262,7 +262,7 @@ protected function execute(InputInterface $input, OutputInterface $output): int
         return 0;
     }
 
-    private function getRecipe(PackageInterface $package, string $recipeRef = null, string $recipeVersion = null): ?Recipe
+    private function getRecipe(PackageInterface $package, ?string $recipeRef = null, ?string $recipeVersion = null): ?Recipe
     {
         $operation = new InformationOperation($package);
         if (null !== $recipeRef) {
diff --git a/src/Configurator/EnvConfigurator.php b/src/Configurator/EnvConfigurator.php
index 9fd81bd30..97c62d6f0 100644
--- a/src/Configurator/EnvConfigurator.php
+++ b/src/Configurator/EnvConfigurator.php
@@ -177,7 +177,7 @@ private function unconfigurePhpUnit(Recipe $recipe, $vars)
      * If $originalValue is passed, and the value contains an expression.
      * the $originalValue is used.
      */
-    private function evaluateValue($value, string $originalValue = null)
+    private function evaluateValue($value, ?string $originalValue = null)
     {
         if ('%generate(secret)%' === $value) {
             if (null !== $originalValue) {
diff --git a/src/Downloader.php b/src/Downloader.php
index d03f6b8b4..fe8aa90e7 100644
--- a/src/Downloader.php
+++ b/src/Downloader.php
@@ -101,7 +101,7 @@ public function getSessionId(): string
         return $this->sess;
     }
 
-    public function setFlexId(string $id = null)
+    public function setFlexId(?string $id = null)
     {
         // No-op to support downgrading to v1.12.x
     }
diff --git a/src/Flex.php b/src/Flex.php
index a305d386f..a45ce90df 100644
--- a/src/Flex.php
+++ b/src/Flex.php
@@ -584,7 +584,7 @@ function ($value) {
         $this->finish($rootDir, $originalComposerJsonHash);
     }
 
-    public function finish(string $rootDir, string $originalComposerJsonHash = null): void
+    public function finish(string $rootDir, ?string $originalComposerJsonHash = null): void
     {
         $this->synchronizePackageJson($rootDir);
         $this->lock->write();
@@ -934,7 +934,7 @@ private function formatOrigin(Recipe $recipe): string
         return sprintf('<info>%s</> (<comment>>=%s</>): From %s', $matches[1], $matches[2], 'auto-generated recipe' === $matches[3] ? '<comment>'.$matches[3].'</>' : $matches[3]);
     }
 
-    private function shouldRecordOperation(OperationInterface $operation, bool $isDevMode, Composer $composer = null): bool
+    private function shouldRecordOperation(OperationInterface $operation, bool $isDevMode, ?Composer $composer = null): bool
     {
         if ($this->dryRun || $this->reinstall) {
             return false;
diff --git a/src/Options.php b/src/Options.php
index f0872b510..3e41f2296 100644
--- a/src/Options.php
+++ b/src/Options.php
@@ -23,7 +23,7 @@ class Options
     private $writtenFiles = [];
     private $io;
 
-    public function __construct(array $options = [], IOInterface $io = null)
+    public function __construct(array $options = [], ?IOInterface $io = null)
     {
         $this->options = $options;
         $this->io = $io;
diff --git a/src/ParallelDownloader.php b/src/ParallelDownloader.php
index 5452df6c3..bfa95d305 100644
--- a/src/ParallelDownloader.php
+++ b/src/ParallelDownloader.php
@@ -216,7 +216,7 @@ public function callbackGet($notificationCode, $severity, $message, $messageCode
     /**
      * {@inheritdoc}
      */
-    protected function getRemoteContents($originUrl, $fileUrl, $context, array &$responseHeaders = null, $maxFileSize = null)
+    protected function getRemoteContents($originUrl, $fileUrl, $context, ?array &$responseHeaders = null, $maxFileSize = null)
     {
         if (isset(self::$cache[$fileUrl])) {
             self::$cacheNext = false;
diff --git a/src/ScriptExecutor.php b/src/ScriptExecutor.php
index d61694735..9aa3327f5 100644
--- a/src/ScriptExecutor.php
+++ b/src/ScriptExecutor.php
@@ -31,7 +31,7 @@ class ScriptExecutor
     private $options;
     private $executor;
 
-    public function __construct(Composer $composer, IOInterface $io, Options $options, ProcessExecutor $executor = null)
+    public function __construct(Composer $composer, IOInterface $io, Options $options, ?ProcessExecutor $executor = null)
     {
         $this->composer = $composer;
         $this->io = $io;
diff --git a/src/TruncatedComposerRepository.php b/src/TruncatedComposerRepository.php
index 82485aeaf..ddf77eb87 100644
--- a/src/TruncatedComposerRepository.php
+++ b/src/TruncatedComposerRepository.php
@@ -23,7 +23,7 @@
  */
 class TruncatedComposerRepository extends BaseComposerRepository
 {
-    public function __construct(array $repoConfig, IOInterface $io, Config $config, EventDispatcher $eventDispatcher = null, RemoteFilesystem $rfs = null)
+    public function __construct(array $repoConfig, IOInterface $io, Config $config, ?EventDispatcher $eventDispatcher = null, ?RemoteFilesystem $rfs = null)
     {
         parent::__construct($repoConfig, $io, $config, $eventDispatcher, $rfs);
 
diff --git a/src/Unpacker.php b/src/Unpacker.php
index 97482929a..ddc0cb189 100644
--- a/src/Unpacker.php
+++ b/src/Unpacker.php
@@ -42,7 +42,7 @@ public function __construct(Composer $composer, PackageResolver $resolver, bool
         $this->versionParser = new VersionParser();
     }
 
-    public function unpack(Operation $op, Result $result = null, &$links = [], bool $devRequire = false): Result
+    public function unpack(Operation $op, ?Result $result = null, &$links = [], bool $devRequire = false): Result
     {
         if (null === $result) {
             $result = new Result();