diff --git a/src/Parser.php b/src/Parser.php
index c46b061..10b6e7a 100644
--- a/src/Parser.php
+++ b/src/Parser.php
@@ -56,7 +56,7 @@ private function upper(string $string) : string
         return mb_convert_case(
             mb_strtolower($string),
             MB_CASE_TITLE,
-            "UTF-8"
+            'UTF-8'
         );
     }
 }