diff --git a/imap_filter/imap_filter.py b/imap_filter/imap_filter.py
index 16824d0..3eaa11d 100644
--- a/imap_filter/imap_filter.py
+++ b/imap_filter/imap_filter.py
@@ -301,7 +301,7 @@ class ImapFilter:
         message = email.message_from_string(part)
         decoded_field = email.header.decode_header(message.get(mfilter['field'], ""))
         for raw_field_data in decoded_field:
-            if isinstance(raw_field_data[0], str):  
+            if isinstance(raw_field_data[0], str):
                 field_data = raw_field_data[0]
             else:
                 try: