--- a/selectors.py	Wed Feb 18 11:46:23 2009 +0100
+++ b/selectors.py	Wed Feb 18 11:46:35 2009 +0100
@@ -384,7 +384,7 @@
                      object to create a relation with another)
     """
     def __init__(self, *expected):
-        assert expected
+        assert expected, self
         self.expected = frozenset(expected)
         
     @lltrace