diff --git a/src/AbstractMessage.php b/src/AbstractMessage.php
index d87bd32..3e40e32 100644
--- a/src/AbstractMessage.php
+++ b/src/AbstractMessage.php
@@ -17,7 +17,7 @@ abstract class AbstractMessage implements Message
      * @param \Protobuf\Stream|resource|string $stream
      * @param \Protobuf\Configuration          $configuration
      */
-    public function __construct($stream = null, \Protobuf\Configuration $configuration = null)
+    public function __construct(?$stream = null, ?\Protobuf\Configuration $configuration = null)
     {
         if ($stream === null) {
             return;
diff --git a/src/Binary/StreamWriter.php b/src/Binary/StreamWriter.php
index 235af8d..6d67119 100644
--- a/src/Binary/StreamWriter.php
+++ b/src/Binary/StreamWriter.php
@@ -50,7 +50,7 @@ public function __construct(Configuration $config)
      * @param string           $bytes
      * @param int              $length
      */
-    public function writeBytes(Stream $stream, $bytes, $length = null)
+    public function writeBytes(Stream $stream, $bytes, ?$length = null)
     {
         if ($length === null) {
             $length = mb_strlen($bytes, '8bit');
@@ -290,7 +290,7 @@ public function writeByteStream(Stream $stream, Stream $value)
      * @param \Protobuf\Stream $value
      * @param int              $length
      */
-    public function writeStream(Stream $stream, Stream $value, $length = null)
+    public function writeStream(Stream $stream, Stream $value, ?$length = null)
     {
         if ($length === null) {
             $length = $value->getSize();
diff --git a/src/Extension/ExtensionField.php b/src/Extension/ExtensionField.php
index 082c9bc..0823b28 100644
--- a/src/Extension/ExtensionField.php
+++ b/src/Extension/ExtensionField.php
@@ -57,7 +57,7 @@ class ExtensionField
      * @param callback $sizeCalculator
      * @param string   $method
      */
-    public function __construct($extendee, $name, $tag, $reader, $writer, $sizeCalculator, $method = null)
+    public function __construct($extendee, $name, $tag, $reader, $writer, $sizeCalculator, ?$method = null)
     {
         $this->tag            = $tag;
         $this->name           = $name;
diff --git a/src/Extension/ExtensionFieldMap.php b/src/Extension/ExtensionFieldMap.php
index e5b32c3..ecced29 100644
--- a/src/Extension/ExtensionFieldMap.php
+++ b/src/Extension/ExtensionFieldMap.php
@@ -26,7 +26,7 @@ class ExtensionFieldMap extends SplObjectStorage implements Collection
     /**
      * @param string $extendee
      */
-    public function __construct($extendee = null)
+    public function __construct(?$extendee = null)
     {
         $this->extendee = trim($extendee, '\\');
     }
diff --git a/src/Message.php b/src/Message.php
index a319f7f..2540d78 100644
--- a/src/Message.php
+++ b/src/Message.php
@@ -22,7 +22,7 @@ interface Message
      *
      * @return \Protobuf\Message
      */
-    public static function fromStream($stream, Configuration $configuration = null);
+    public static function fromStream($stream, Configuration ?$configuration = null);
 
     /**
      * Serializes the message and returns a stream containing its bytes.
@@ -31,7 +31,7 @@ public static function fromStream($stream, Configuration $configuration = null);
      *
      * @return \Protobuf\Stream
      */
-    public function toStream(Configuration $configuration = null);
+    public function toStream(Configuration ?$configuration = null);
 
     /**
      * Compute the number of bytes that would be needed to encode the message
diff --git a/src/MessageSerializer.php b/src/MessageSerializer.php
index 23df67f..58c17fb 100644
--- a/src/MessageSerializer.php
+++ b/src/MessageSerializer.php
@@ -17,7 +17,7 @@ class MessageSerializer implements Serializer
     /**
      * @param \Protobuf\Configuration $config
      */
-    public function __construct(Configuration $config = null)
+    public function __construct(Configuration ?$config = null)
     {
         $this->config = $config ?: Configuration::getInstance();
     }
diff --git a/src/ReadContext.php b/src/ReadContext.php
index 5099f63..d8eaa99 100644
--- a/src/ReadContext.php
+++ b/src/ReadContext.php
@@ -38,7 +38,7 @@ class ReadContext
      * @param \Protobuf\Binary\StreamReader         $reader
      * @param \Protobuf\Extension\ExtensionRegistry $extensionRegistry
      */
-    public function __construct($stream, StreamReader $reader, ExtensionRegistry $extensionRegistry = null)
+    public function __construct($stream, StreamReader $reader, ExtensionRegistry ?$extensionRegistry = null)
     {
         if ( ! $stream instanceof \Protobuf\Stream) {
             $stream = Stream::wrap($stream);
diff --git a/src/Stream.php b/src/Stream.php
index 4c7b5b1..454e8d2 100644
--- a/src/Stream.php
+++ b/src/Stream.php
@@ -28,7 +28,7 @@ class Stream
      *
      * @throws \InvalidArgumentException if the stream is not a stream resource
      */
-    public function __construct($stream, $size = null)
+    public function __construct($stream, ?$size = null)
     {
         if ( ! is_resource($stream)) {
             throw new InvalidArgumentException('Stream must be a resource');
@@ -253,7 +253,7 @@ public function writeStream(Stream $stream, $length)
      *
      * @throws \InvalidArgumentException if the $resource arg is not valid.
      */
-    public static function wrap($resource = '', $size = null)
+    public static function wrap($resource = '', ?$size = null)
     {
         if ($resource instanceof Stream) {
             return $resource;
@@ -290,7 +290,7 @@ public static function create()
      *
      * @return \Protobuf\Stream
      */
-    public static function fromString($resource = '', $size = null)
+    public static function fromString($resource = '', ?$size = null)
     {
         $stream = fopen('php://temp', 'r+');
 
diff --git a/src/Unknown.php b/src/Unknown.php
index ccfa28e..2b78ab6 100644
--- a/src/Unknown.php
+++ b/src/Unknown.php
@@ -30,7 +30,7 @@ class Unknown
      * @param integer $type
      * @param mixed   $value
      */
-    public function __construct($tag = 0, $type = null, $value = null)
+    public function __construct($tag = 0, $type = null, ?$value = null)
     {
         $this->tag   = $tag;
         $this->type  = $type;