diff --git a/imbox/parser.py b/imbox/parser.py
index 9ad1314..58e10bf 100644
--- a/imbox/parser.py
+++ b/imbox/parser.py
@@ -33,7 +33,10 @@ def decode_mail_header(value, default_charset='us-ascii'):
         return str_decode(str_encode(value, default_charset, 'replace'), default_charset)
     else:
         for index, (text, charset) in enumerate(headers):
-            logger.debug("Mail header no. {}: {} encoding {}".format(index, str_decode(text, charset or 'utf-8', 'replace'), charset))
+            logger.debug("Mail header no. {index}: {data} encoding {charset}".format(
+                index=index,
+                data=str_decode(text, charset or 'utf-8', 'replace'),
+                charset=charset))
             try:
                 headers[index] = str_decode(text, charset or default_charset,
                                             'replace')