diff --git a/src/JsonFormatter.php b/src/JsonFormatter.php
index 069a5fd1..395503ad 100644
--- a/src/JsonFormatter.php
+++ b/src/JsonFormatter.php
@@ -29,7 +29,7 @@ final class JsonFormatter
      *
      * @return string
      */
-    public static function format($json, $unescapeUnicode, $unescapeSlashes)
+    public static function format(string $json, bool $unescapeUnicode, bool $unescapeSlashes): string
     {
         $result = '';
         $pos = 0;
@@ -63,7 +63,7 @@ public static function format($json, $unescapeUnicode, $unescapeSlashes)
 
                 if ($unescapeUnicode && \function_exists('mb_convert_encoding')) {
                     // https://stackoverflow.com/questions/2934563/how-to-decode-unicode-escape-sequences-like-u00ed-to-proper-utf-8-encoded-cha
-                    $buffer = \preg_replace_callback('/(\\\\+)u([0-9a-f]{4})/i', function ($match) {
+                    $buffer = \preg_replace_callback('/(\\\\+)u([0-9a-f]{4})/i', function (array $match) {
                         $l = \strlen($match[1]);
 
                         if ($l % 2) {