[Zope] "Missing" attribute with ZSQL methods?

If I write DTML that way: <td><dtml-var telephone missing="(No phone)"></td> and if "telephone" comes from a ZSQL method, "missing" is ignored even if the field was NULL (in SQL sense). Is it normal? Zope version: Zope 2.2.2 (source release, python 1.5.2, linux2) Python version: 2.0c1 (#2, Nov 24 2000, 11:08:10) [C] System Platform: osf1V4 PostgreSQL 7.0.3 (with Z PyGreSQL, ZPyGreSQLDA-0-0-3)