diff --git a/scrapy/contrib/spidermiddleware/offsite.py b/scrapy/contrib/spidermiddleware/offsite.py
index 95702d777..136714508 100644
--- a/scrapy/contrib/spidermiddleware/offsite.py
+++ b/scrapy/contrib/spidermiddleware/offsite.py
@@ -49,7 +49,7 @@ class OffsiteMiddleware(object):
         allowed_domains = getattr(spider, 'allowed_domains', None)
         if not allowed_domains:
             return re.compile('') # allow all by default
-        regex = r'^(.*\.)?(%s)$' % '|'.join(re.escape(d) for d in allowed_domains)
+        regex = r'^(.*\.)?(%s)$' % '|'.join(re.escape(d) for d in allowed_domains if d is not None)
         return re.compile(regex)
 
     def spider_opened(self, spider):