diff --git a/src/Converter.php b/src/Converter.php
index 54d32d41..fdbbcefd 100644
--- a/src/Converter.php
+++ b/src/Converter.php
@@ -238,8 +238,8 @@ protected function formatTone(string $pinyin, string $style): string
         }
 
         $replacements = [
-            // chr(201) => 'ɑ' 轻声中除了 `ɑ` 和 `ü` 以外，其它和字母一样
-            'ɑ' => [chr(201), 5], 'ü' => ['v', 5],
+            // mb_chr(593) => 'ɑ' 轻声中除了 `ɑ` 和 `ü` 以外，其它和字母一样
+            'ɑ' => ['a', 5], 'ü' => ['v', 5],
             'üē' => ['ue', 1], 'üé' => ['ue', 2], 'üě' => ['ue', 3], 'üè' => ['ue', 4],
             'ā' => ['a', 1], 'ē' => ['e', 1], 'ī' => ['i', 1], 'ō' => ['o', 1], 'ū' => ['u', 1], 'ǖ' => ['v', 1],
             'á' => ['a', 2], 'é' => ['e', 2], 'í' => ['i', 2], 'ó' => ['o', 2], 'ú' => ['u', 2], 'ǘ' => ['v', 2],