--- a/_exceptions.py	Wed Apr 22 12:05:09 2015 +0200
+++ b/_exceptions.py	Wed Jan 28 15:53:41 2015 +0100
@@ -82,6 +82,8 @@
         self.session = session
         assert 'rtypes' in kwargs or 'cstrname' in kwargs
         self.kwargs = kwargs
+        # fill cache while the session is open
+        self.rtypes
 
     @cachedproperty
     def rtypes(self):