diff --git a/openerp/addons/base/ir/ir_attachment.py b/openerp/addons/base/ir/ir_attachment.py
index 2dffe9d0b86..73a8867a98d 100644
--- a/openerp/addons/base/ir/ir_attachment.py
+++ b/openerp/addons/base/ir/ir_attachment.py
@@ -255,9 +255,8 @@ class ir_attachment(osv.osv):
         """ compute the checksum for the given datas
             :param bin_data : datas in its binary form
         """
-        if bin_data:
-            return hashlib.sha1(bin_data).hexdigest()
-        return False
+        # an empty file has a checksum too (for caching)
+        return hashlib.sha1(bin_data or '').hexdigest()
 
     def _compute_mimetype(self, values):
         """ compute the mimetype of the given values
diff --git a/openerp/addons/base/ir/ir_http.py b/openerp/addons/base/ir/ir_http.py
index cc2e01890b2..48479528bd0 100644
--- a/openerp/addons/base/ir/ir_http.py
+++ b/openerp/addons/base/ir/ir_http.py
@@ -107,6 +107,7 @@ class ir_http(osv.AbstractModel):
             wdate = attach[0]['__last_update']
             datas = attach[0]['datas'] or ''
             name = attach[0]['name']
+            checksum = attach[0]['checksum'] or hashlib.sha1(datas).hexdigest()
 
             if (not datas and name != request.httprequest.path and
                     name.startswith(('http://', 'https://', '/'))):
@@ -120,7 +121,7 @@ class ir_http(osv.AbstractModel):
                 # just in case we have a timestamp without microseconds
                 response.last_modified = datetime.datetime.strptime(wdate, server_format)
 
-            response.set_etag(attach[0]['checksum'])
+            response.set_etag(checksum)
             response.make_conditional(request.httprequest)
 
             if response.status_code == 304: