diff --git a/Mbstring.php b/Mbstring.php
index 3d45c9d..31e36a3 100644
--- a/Mbstring.php
+++ b/Mbstring.php
@@ -983,7 +983,7 @@ public static function mb_ltrim(string $string, ?string $characters = null, ?str
 
     public static function mb_rtrim(string $string, ?string $characters = null, ?string $encoding = null): string
     {
-        return self::mb_internal_trim('{[%s]+$}D', $string, $characters, $encoding, __FUNCTION__);
+        return self::mb_internal_trim('{[%s]+$}Du', $string, $characters, $encoding, __FUNCTION__);
     }
 
     private static function mb_internal_trim(string $regex, string $string, ?string $characters, ?string $encoding, string $function): string
diff --git a/bootstrap80.php b/bootstrap80.php
index 5be7d20..5236e6d 100644
--- a/bootstrap80.php
+++ b/bootstrap80.php
@@ -133,11 +133,11 @@ function mb_str_pad(string $string, int $length, string $pad_string = ' ', int $
 }
 
 if (!function_exists('mb_ucfirst')) {
-    function mb_ucfirst($string, ?string $encoding = null): string { return p\Mbstring::mb_ucfirst($string, $encoding); }
+    function mb_ucfirst(string $string, ?string $encoding = null): string { return p\Mbstring::mb_ucfirst($string, $encoding); }
 }
 
 if (!function_exists('mb_lcfirst')) {
-    function mb_lcfirst($string, ?string $encoding = null): string { return p\Mbstring::mb_lcfirst($string, $encoding); }
+    function mb_lcfirst(string $string, ?string $encoding = null): string { return p\Mbstring::mb_lcfirst($string, $encoding); }
 }
 
 if (!function_exists('mb_trim')) {
diff --git a/composer.json b/composer.json
index 4ed241a..daa07f8 100644
--- a/composer.json
+++ b/composer.json
@@ -16,7 +16,8 @@
         }
     ],
     "require": {
-        "php": ">=7.2"
+        "php": ">=7.2",
+        "ext-iconv": "*"
     },
     "provide": {
         "ext-mbstring": "*"