a.diff

diff --git a/scrapy/log.py b/scrapy/log.py
index be2c1fd..83be049 100644
--- a/scrapy/log.py
+++ b/scrapy/log.py
@@ -1,4 +1,4 @@
-""" 
+"""
 Scrapy logging facility
 
 See documentation in docs/topics/logging.rst
@@ -7,11 +7,11 @@ import sys
 import logging
 import warnings
 
-from twisted.python import log
+from twisted.python import log, _safeFormat
 
 import scrapy
 from scrapy.utils.python import unicode_to_str
- 
+
 # Logging levels
 DEBUG = logging.DEBUG
 INFO = logging.INFO
@@ -131,6 +131,8 @@ def msg(message=None, _level=INFO, **kw):
 def err(_stuff=None, _why=None, **kw):
     kw['logLevel'] = kw.pop('level', ERROR)
     kw.setdefault('system', 'scrapy')
+    if _why is None and 'format' in kw:
+        _why = _safeFormat(kw['format'], kw)
     log.err(_stuff, _why, **kw)
 
 def start_from_crawler(crawler):