--- a/cubicweb/server/session.py	Fri Oct 21 18:35:46 2016 +0200
+++ b/cubicweb/server/session.py	Wed Oct 12 17:03:02 2016 +0200
@@ -381,7 +381,7 @@
     # life cycle handling ####################################################
 
     def __enter__(self):
-        assert self._open is None  # first opening
+        assert not self._open
         self._open = True
         self.cnxset = self.repo._get_cnxset()
         return self