diff --git a/openerp/service/security.py b/openerp/service/security.py
index 28c85788a0a..babdd88f30e 100644
--- a/openerp/service/security.py
+++ b/openerp/service/security.py
@@ -18,7 +18,8 @@ def compute_session_token(session, env):
 
 def check_session(session, env):
     self = env['res.users'].browse(session.uid)
-    if openerp.tools.misc.consteq(self._compute_session_token(session.sid), session.session_token):
+    expected = self._compute_session_token(session.sid)
+    if expected and openerp.tools.misc.consteq(expected, session.session_token):
         return True
     self._invalidate_session_cache()
     return False