diff --git a/scrapy/spidermiddlewares/offsite.py b/scrapy/spidermiddlewares/offsite.py
index 3b7f194e4..310166cad 100644
--- a/scrapy/spidermiddlewares/offsite.py
+++ b/scrapy/spidermiddlewares/offsite.py
@@ -19,9 +19,6 @@ class OffsiteMiddleware(object):
 
     def __init__(self, stats):
         self.stats = stats
-        # default values
-        self.host_regex = re.compile('')  # allow all by default
-        self.domains_seen = set()
 
     @classmethod
     def from_crawler(cls, crawler):
@@ -55,7 +52,7 @@ class OffsiteMiddleware(object):
         """Override this method to implement a different offsite policy"""
         allowed_domains = getattr(spider, 'allowed_domains', None)
         if not allowed_domains:
-            return
+            return re.compile('') # allow all by default
         url_pattern = re.compile("^https?://.*$")
         for domain in allowed_domains:
             if url_pattern.match(domain):
@@ -65,9 +62,8 @@ class OffsiteMiddleware(object):
         return re.compile(regex)
 
     def spider_opened(self, spider):
-        host_regex = self.get_host_regex(spider)
-        if host_regex:
-            self.host_regex = host_regex
+        self.host_regex = self.get_host_regex(spider)
+        self.domains_seen = set()
 
 
 class URLWarning(Warning):