diff --git a/src/Exception/InvalidCodePointException.php b/src/Exception/InvalidCodePointException.php
index f1a3fe9..9494adc 100644
--- a/src/Exception/InvalidCodePointException.php
+++ b/src/Exception/InvalidCodePointException.php
@@ -30,7 +30,7 @@ class InvalidCodePointException extends UnicodeException
      * @param $codePoint
      * @param Throwable|null $previous
      */
-    public function __construct($codePoint, Throwable $previous = null)
+    public function __construct($codePoint, ?Throwable $previous = null)
     {
         parent::__construct("Invalid code point", 0, $previous);
         $this->codePoint = $codePoint;
diff --git a/src/Exception/InvalidStringException.php b/src/Exception/InvalidStringException.php
index 11e52dd..50b22a1 100644
--- a/src/Exception/InvalidStringException.php
+++ b/src/Exception/InvalidStringException.php
@@ -36,7 +36,7 @@ class InvalidStringException extends UnicodeException
      * @param int $offset
      * @param Throwable|null $previous
      */
-    public function __construct(string $string, int $offset = -1, Throwable $previous = null)
+    public function __construct(string $string, int $offset = -1, ?Throwable $previous = null)
     {
         parent::__construct("Invalid UTF-8 string at offset {$offset}", 0, $previous);
         $this->string = $string;