diff --git a/src/Common/BitMatrix.php b/src/Common/BitMatrix.php
index fc0d1f1..294afb4 100644
--- a/src/Common/BitMatrix.php
+++ b/src/Common/BitMatrix.php
@@ -40,7 +40,7 @@ class BitMatrix
     /**
      * @throws InvalidArgumentException if a dimension is smaller than zero
      */
-    public function __construct(int $width, int $height = null)
+    public function __construct(int $width, ?int $height = null)
     {
         if (null === $height) {
             $height = $width;
@@ -132,7 +132,7 @@ public function setRegion(int $left, int $top, int $width, int $height) : void
     /**
      * A fast method to retrieve one row of data from the matrix as a BitArray.
      */
-    public function getRow(int $y, BitArray $row = null) : BitArray
+    public function getRow(int $y, ?BitArray $row = null) : BitArray
     {
         if (null === $row || $row->getSize() < $this->width) {
             $row = new BitArray($this->width);
diff --git a/src/Common/ReedSolomonCodec.php b/src/Common/ReedSolomonCodec.php
index 31d8030..d16a75e 100644
--- a/src/Common/ReedSolomonCodec.php
+++ b/src/Common/ReedSolomonCodec.php
@@ -208,7 +208,7 @@ public function encode(SplFixedArray $data, SplFixedArray $parity) : void
     /**
      * Decodes received data.
      */
-    public function decode(SplFixedArray $data, SplFixedArray $erasures = null) : ?int
+    public function decode(SplFixedArray $data, ?SplFixedArray $erasures = null) : ?int
     {
         // This speeds up the initialization a bit.
         $numRootsPlusOne = SplFixedArray::fromArray(array_fill(0, $this->numRoots + 1, 0), false);
diff --git a/src/Encoder/Encoder.php b/src/Encoder/Encoder.php
index 5444c28..c363953 100644
--- a/src/Encoder/Encoder.php
+++ b/src/Encoder/Encoder.php
@@ -160,7 +160,7 @@ private static function getAlphanumericCode(int $code) : int
     /**
      * Chooses the best mode for a given content.
      */
-    private static function chooseMode(string $content, string $encoding = null) : Mode
+    private static function chooseMode(string $content, ?string $encoding = null) : Mode
     {
         if (null !== $encoding && 0 === strcasecmp($encoding, 'SHIFT-JIS')) {
             return self::isOnlyDoubleByteKanji($content) ? Mode::KANJI() : Mode::BYTE();