diff --git a/oscad/__init__.py b/oscad/__init__.py
index 71a3b2d..4010f54 100644
--- a/oscad/__init__.py
+++ b/oscad/__init__.py
@@ -121,7 +121,7 @@ def main(global_config, **settings):
     config.set_session_factory(SignedCookieSessionFactory(
         '',
         httponly=True,
-        serializer=JSONSerializer))
+        serializer=JSONSerializer()))
     config.add_subscriber('oscad.subscribers.add_renderer_globals',
                           'pyramid.events.BeforeRender')
     config.add_subscriber('oscad.subscribers.jsonify',