diff --git a/addons/web/http.py b/addons/web/http.py
index beac395c8a1..de3fb1386e6 100644
--- a/addons/web/http.py
+++ b/addons/web/http.py
@@ -827,7 +827,7 @@ class Root(object):
             m = registry.get('ir.module.module')
             ids = m.search(cr, openerp.SUPERUSER_ID, [('state','=','installed')])
             installed = set([x['name'] for x in m.read(cr, 1, ids, ['name'])])
-            modules_set -= set(installed)
+            modules_set = modules_set.intersection(set(installed))
         modules = ["web"] + sorted(modules_set)
         # building all other methods
         for module in modules: