diff --git a/imap_filter/imap_filter.py b/imap_filter/imap_filter.py
index b25f84b..2a7ef24 100644
--- a/imap_filter/imap_filter.py
+++ b/imap_filter/imap_filter.py
@@ -135,7 +135,7 @@ class ImapFilter:
         try:
             part = data[1].decode('utf-8')
         except UnicodeDecodeError:
-            part = data[1]
+            part = str(data[1])
         message = email.message_from_string(part)
         decoded_field = email.header.decode_header(message.get(mfilter['field'], ""))
         if isinstance(decoded_field[0][0], str):