diff --git a/src/Illuminate/Support/Str.php b/src/Illuminate/Support/Str.php
index c1cc95f2fbc9..da9de3b9f002 100644
--- a/src/Illuminate/Support/Str.php
+++ b/src/Illuminate/Support/Str.php
@@ -297,7 +297,7 @@ public static function containsAll($haystack, $needles, $ignoreCase = false)
      *
      * @param  string  $string
      * @param  int  $mode
-     * @param  string  $encoding
+     * @param  string|null  $encoding
      * @return string
      */
     public static function convertCase(string $string, int $mode = MB_CASE_FOLD, ?string $encoding = 'UTF-8')