diff --git a/lib/CancellationTokenSource.php b/lib/CancellationTokenSource.php
index a57c6e9e..965c7cb0 100644
--- a/lib/CancellationTokenSource.php
+++ b/lib/CancellationTokenSource.php
@@ -150,7 +150,7 @@ public function getToken(): CancellationToken
      *
      * @return void
      */
-    public function cancel(\Throwable $previous = null)
+    public function cancel(?\Throwable $previous = null)
     {
         if ($this->onCancel === null) {
             return;
diff --git a/lib/CancelledException.php b/lib/CancelledException.php
index 25f8c400..7cc1da46 100644
--- a/lib/CancelledException.php
+++ b/lib/CancelledException.php
@@ -10,7 +10,7 @@
  */
 class CancelledException extends \Exception
 {
-    public function __construct(\Throwable $previous = null)
+    public function __construct(?\Throwable $previous = null)
     {
         parent::__construct("The operation was cancelled", 0, $previous);
     }
diff --git a/lib/Coroutine.php b/lib/Coroutine.php
index 5a3b4aa8..d4195bfe 100644
--- a/lib/Coroutine.php
+++ b/lib/Coroutine.php
@@ -88,7 +88,7 @@ public function __construct(\Generator $generator)
          * @psalm-suppress MissingClosureParamType
          * @psalm-suppress MissingClosureReturnType
          */
-        $onResolve = function (\Throwable $e = null, $v) use ($generator, &$onResolve) {
+        $onResolve = function (?\Throwable $e = null, $v) use ($generator, &$onResolve) {
             /** @var bool $immediate Used to control iterative coroutine continuation. */
             static $immediate = true;
 
diff --git a/lib/Internal/ResolutionQueue.php b/lib/Internal/ResolutionQueue.php
index 353a8b93..8b6397e3 100644
--- a/lib/Internal/ResolutionQueue.php
+++ b/lib/Internal/ResolutionQueue.php
@@ -27,7 +27,7 @@ class ResolutionQueue
      *     Promise|\React\Promise\PromiseInterface|array<array-key, Promise|\React\Promise\PromiseInterface>, mixed,
      *     mixed>|null) | callable(\Throwable|null, mixed): void $callback
      */
-    public function __construct(callable $callback = null)
+    public function __construct(?callable $callback = null)
     {
         if ($callback !== null) {
             $this->push($callback);
diff --git a/lib/InvalidYieldError.php b/lib/InvalidYieldError.php
index 8785708e..fddb7710 100644
--- a/lib/InvalidYieldError.php
+++ b/lib/InvalidYieldError.php
@@ -9,7 +9,7 @@ class InvalidYieldError extends \Error
      * @param string          $prefix
      * @param \Throwable|null $previous
      */
-    public function __construct(\Generator $generator, string $prefix, \Throwable $previous = null)
+    public function __construct(\Generator $generator, string $prefix, ?\Throwable $previous = null)
     {
         $yielded = $generator->current();
         $prefix .= \sprintf(
diff --git a/lib/Loop.php b/lib/Loop.php
index d89d2101..33adce10 100644
--- a/lib/Loop.php
+++ b/lib/Loop.php
@@ -86,7 +86,7 @@ public function getHandle()
      *
      * @return void
      */
-    public static function run(callable $callback = null)
+    public static function run(?callable $callback = null)
     {
         if ($callback) {
             self::$driver->defer($callback);
@@ -406,7 +406,7 @@ public static function getState(string $key)
      *
      * @return callable(\Throwable $error)|null The previous handler, `null` if there was none.
      */
-    public static function setErrorHandler(callable $callback = null)
+    public static function setErrorHandler(?callable $callback = null)
     {
         return self::$driver->setErrorHandler($callback);
     }
diff --git a/lib/Loop/Driver.php b/lib/Loop/Driver.php
index 448bab62..89b372a7 100644
--- a/lib/Loop/Driver.php
+++ b/lib/Loop/Driver.php
@@ -593,7 +593,7 @@ final public function getState(string $key)
      *
      * @return callable(\Throwable $error):void|null The previous handler, `null` if there was none.
      */
-    public function setErrorHandler(callable $callback = null)
+    public function setErrorHandler(?callable $callback = null)
     {
         $previous = $this->errorHandler;
         $this->errorHandler = $callback;
diff --git a/lib/Loop/TracingDriver.php b/lib/Loop/TracingDriver.php
index 7b787548..3e7de532 100644
--- a/lib/Loop/TracingDriver.php
+++ b/lib/Loop/TracingDriver.php
@@ -147,7 +147,7 @@ public function unreference(string $watcherId)
         $this->unreferencedWatchers[$watcherId] = true;
     }
 
-    public function setErrorHandler(callable $callback = null)
+    public function setErrorHandler(?callable $callback = null)
     {
         return $this->driver->setErrorHandler($callback);
     }
diff --git a/lib/MultiReasonException.php b/lib/MultiReasonException.php
index 19c5c4d8..09758fb8 100644
--- a/lib/MultiReasonException.php
+++ b/lib/MultiReasonException.php
@@ -11,7 +11,7 @@ class MultiReasonException extends \Exception
      * @param \Throwable[] $reasons Array of exceptions rejecting the promise.
      * @param string|null  $message
      */
-    public function __construct(array $reasons, string $message = null)
+    public function __construct(array $reasons, ?string $message = null)
     {
         parent::__construct($message ?: "Multiple errors encountered; use "
             . self::class . "::getReasons() to retrieve the array of exceptions thrown");
diff --git a/lib/functions.php b/lib/functions.php
index 7b1c2c55..31f41283 100644
--- a/lib/functions.php
+++ b/lib/functions.php
@@ -570,7 +570,7 @@ function wrap($promise, callable $callback): Promise
 
         $deferred = new Deferred();
 
-        $promise->onResolve(static function (\Throwable $exception = null, $result) use ($deferred, $callback) {
+        $promise->onResolve(static function (?\Throwable $exception = null, $result) use ($deferred, $callback) {
             try {
                 $result = $callback($exception, $result);
             } catch (\Throwable $exception) {