diff --git a/ExecutableFinder.php b/ExecutableFinder.php
index eb8f0629..f392c962 100644
--- a/ExecutableFinder.php
+++ b/ExecutableFinder.php
@@ -46,7 +46,7 @@ public function addSuffix(string $suffix)
      *
      * @return string|null
      */
-    public function find(string $name, string $default = null, array $extraDirs = [])
+    public function find(string $name, ?string $default = null, array $extraDirs = [])
     {
         if (\ini_get('open_basedir')) {
             $searchPath = array_merge(explode(\PATH_SEPARATOR, \ini_get('open_basedir')), $extraDirs);
diff --git a/InputStream.php b/InputStream.php
index 240665f3..0c45b524 100644
--- a/InputStream.php
+++ b/InputStream.php
@@ -30,7 +30,7 @@ class InputStream implements \IteratorAggregate
     /**
      * Sets a callback that is called when the write buffer becomes empty.
      */
-    public function onEmpty(callable $onEmpty = null)
+    public function onEmpty(?callable $onEmpty = null)
     {
         $this->onEmpty = $onEmpty;
     }
diff --git a/PhpProcess.php b/PhpProcess.php
index 2bc338e5..3a1d147c 100644
--- a/PhpProcess.php
+++ b/PhpProcess.php
@@ -32,7 +32,7 @@ class PhpProcess extends Process
      * @param int         $timeout The timeout in seconds
      * @param array|null  $php     Path to the PHP binary to use with any additional arguments
      */
-    public function __construct(string $script, string $cwd = null, array $env = null, int $timeout = 60, array $php = null)
+    public function __construct(string $script, ?string $cwd = null, ?array $env = null, int $timeout = 60, ?array $php = null)
     {
         if (null === $php) {
             $executableFinder = new PhpExecutableFinder();
@@ -53,7 +53,7 @@ public function __construct(string $script, string $cwd = null, array $env = nul
     /**
      * {@inheritdoc}
      */
-    public static function fromShellCommandline(string $command, string $cwd = null, array $env = null, $input = null, ?float $timeout = 60)
+    public static function fromShellCommandline(string $command, ?string $cwd = null, ?array $env = null, $input = null, ?float $timeout = 60)
     {
         throw new LogicException(sprintf('The "%s()" method cannot be called when using "%s".', __METHOD__, self::class));
     }
@@ -61,7 +61,7 @@ public static function fromShellCommandline(string $command, string $cwd = null,
     /**
      * {@inheritdoc}
      */
-    public function start(callable $callback = null, array $env = [])
+    public function start(?callable $callback = null, array $env = [])
     {
         if (null === $this->getCommandLine()) {
             throw new RuntimeException('Unable to find the PHP executable.');
diff --git a/Process.php b/Process.php
index 30ebeb6b..2b6ed9ef 100644
--- a/Process.php
+++ b/Process.php
@@ -140,7 +140,7 @@ class Process implements \IteratorAggregate
      *
      * @throws LogicException When proc_open is not installed
      */
-    public function __construct(array $command, string $cwd = null, array $env = null, $input = null, ?float $timeout = 60)
+    public function __construct(array $command, ?string $cwd = null, ?array $env = null, $input = null, ?float $timeout = 60)
     {
         if (!\function_exists('proc_open')) {
             throw new LogicException('The Process class relies on proc_open, which is not available on your PHP installation.');
@@ -189,7 +189,7 @@ public function __construct(array $command, string $cwd = null, array $env = nul
      *
      * @throws LogicException When proc_open is not installed
      */
-    public static function fromShellCommandline(string $command, string $cwd = null, array $env = null, $input = null, ?float $timeout = 60)
+    public static function fromShellCommandline(string $command, ?string $cwd = null, ?array $env = null, $input = null, ?float $timeout = 60)
     {
         $process = new static([], $cwd, $env, $input, $timeout);
         $process->commandline = $command;
@@ -247,7 +247,7 @@ public function __clone()
      *
      * @final
      */
-    public function run(callable $callback = null, array $env = []): int
+    public function run(?callable $callback = null, array $env = []): int
     {
         $this->start($callback, $env);
 
@@ -266,7 +266,7 @@ public function run(callable $callback = null, array $env = []): int
      *
      * @final
      */
-    public function mustRun(callable $callback = null, array $env = []): self
+    public function mustRun(?callable $callback = null, array $env = []): self
     {
         if (0 !== $this->run($callback, $env)) {
             throw new ProcessFailedException($this);
@@ -294,7 +294,7 @@ public function mustRun(callable $callback = null, array $env = []): self
      * @throws RuntimeException When process is already running
      * @throws LogicException   In case a callback is provided and output has been disabled
      */
-    public function start(callable $callback = null, array $env = [])
+    public function start(?callable $callback = null, array $env = [])
     {
         if ($this->isRunning()) {
             throw new RuntimeException('Process is already running.');
@@ -385,7 +385,7 @@ public function start(callable $callback = null, array $env = [])
      *
      * @final
      */
-    public function restart(callable $callback = null, array $env = []): self
+    public function restart(?callable $callback = null, array $env = []): self
     {
         if ($this->isRunning()) {
             throw new RuntimeException('Process is already running.');
@@ -412,7 +412,7 @@ public function restart(callable $callback = null, array $env = []): self
      * @throws ProcessSignaledException When process stopped after receiving signal
      * @throws LogicException           When process is not yet started
      */
-    public function wait(callable $callback = null)
+    public function wait(?callable $callback = null)
     {
         $this->requireProcessIsStarted(__FUNCTION__);
 
@@ -914,7 +914,7 @@ public function getStatus()
      *
      * @return int|null The exit-code of the process or null if it's not running
      */
-    public function stop(float $timeout = 10, int $signal = null)
+    public function stop(float $timeout = 10, ?int $signal = null)
     {
         $timeoutMicro = microtime(true) + $timeout;
         if ($this->isRunning()) {
@@ -1310,7 +1310,7 @@ private function getDescriptors(): array
      *
      * @return \Closure
      */
-    protected function buildCallback(callable $callback = null)
+    protected function buildCallback(?callable $callback = null)
     {
         if ($this->outputDisabled) {
             return function ($type, $data) use ($callback): bool {
diff --git a/Tests/ProcessTest.php b/Tests/ProcessTest.php
index 80493799..daf842e1 100644
--- a/Tests/ProcessTest.php
+++ b/Tests/ProcessTest.php
@@ -1558,7 +1558,7 @@ public function testNotTerminableInputPipe()
      * @param string|array $commandline
      * @param mixed        $input
      */
-    private function getProcess($commandline, string $cwd = null, array $env = null, $input = null, ?int $timeout = 60): Process
+    private function getProcess($commandline, ?string $cwd = null, ?array $env = null, $input = null, ?int $timeout = 60): Process
     {
         if (\is_string($commandline)) {
             $process = Process::fromShellCommandline($commandline, $cwd, $env, $input, $timeout);
@@ -1573,7 +1573,7 @@ private function getProcess($commandline, string $cwd = null, array $env = null,
         return self::$process = $process;
     }
 
-    private function getProcessForCode(string $code, string $cwd = null, array $env = null, $input = null, ?int $timeout = 60): Process
+    private function getProcessForCode(string $code, ?string $cwd = null, ?array $env = null, $input = null, ?int $timeout = 60): Process
     {
         return $this->getProcess([self::$phpBin, '-r', $code], $cwd, $env, $input, $timeout);
     }