diff -r cc4b8021bdf3 -r 365e5a0287d6 schema.py
--- a/schema.py	Mon Feb 02 22:05:00 2015 +0100
+++ b/schema.py	Mon Feb 02 23:17:15 2015 +0100
@@ -1072,6 +1072,7 @@
         for rdef in self.iter_computed_attributes():
             rqlst = parse(rdef.formula)
             select = rqlst.children[0]
+            select.add_type_restriction(select.defined_vars['X'], str(rdef.subject))
             analyzer.visit(select)
             _check_valid_formula(rdef, rqlst)
             rdef.formula_select = select # avoid later recomputation