diff --git a/scrapy/trunk/scrapy/core/downloader/responsetypes/__init__.py b/scrapy/trunk/scrapy/core/downloader/responsetypes/__init__.py
index 89a39fe8b..32550ad3f 100644
--- a/scrapy/trunk/scrapy/core/downloader/responsetypes/__init__.py
+++ b/scrapy/trunk/scrapy/core/downloader/responsetypes/__init__.py
@@ -31,7 +31,10 @@ class ResponseTypes(object):
 
     def from_mimetype(self, mimetype):
         """Return the most appropiate Response class for the given mimetype"""
-        return self.classes.get(mimetype, self.classes.get(mimetype.split('/')[0], Response))
+        if mimetype is not None:
+            return self.classes.get(mimetype, self.classes.get(mimetype.split('/')[0], Response))
+        else:
+            return Response
 
     def from_content_type(self, content_type):
         """Return the most appropiate Response class from an HTTP Content-Type
diff --git a/scrapy/trunk/scrapy/tests/test_responsetypes.py b/scrapy/trunk/scrapy/tests/test_responsetypes.py
index 5a7fd66e8..76c4e7822 100644
--- a/scrapy/trunk/scrapy/tests/test_responsetypes.py
+++ b/scrapy/trunk/scrapy/tests/test_responsetypes.py
@@ -65,6 +65,9 @@ class ResponseTypesTest(unittest.TestCase):
             ({'url': 'http://www.example.com/data.csv'}, TextResponse),
             # headers takes precedence over url
             ({'headers': Headers({'Content-Type': ['text/html; charset=utf-8']}), 'url': 'http://www.example.com/item/'}, HtmlResponse),
+            ({'headers': Headers({'Content-Disposition': ['attachment; filename="data.xml.gz"']}), 'url': 'http://www.example.com/page/'}, Response),
+
+
         ]
         for source, cls in mappings:
             retcls = responsetypes.from_args(**source)