diff --git a/src/Header/ContentTransferEncoding.php b/src/Header/ContentTransferEncoding.php
index 2734ce07b5..b531489b89 100644
--- a/src/Header/ContentTransferEncoding.php
+++ b/src/Header/ContentTransferEncoding.php
@@ -28,7 +28,7 @@ public static function fromString($headerLine)
         }
 
         // @todo implementation details
-        $header = new static($value);
+        $header = new static(strtolower($value));
 
         return $header;
     }