diff --git a/backup_imap/backup_imap.py b/backup_imap/backup_imap.py
index 95f6d12..24df7da 100644
--- a/backup_imap/backup_imap.py
+++ b/backup_imap/backup_imap.py
@@ -174,7 +174,7 @@ class BackupImap:
         return False
 
     def _get_mail_header(self, header, data):
-        message = email.message_from_string(data)
+        message = email.message_from_bytes(data)
         decoded_header = email.header.decode_header(message.get(header, ""))
         result = []
         for raw_header_data in decoded_header: