[ZPT] request "user" crashed the TAL page

[iap@y2fun.com]

Hi,
I got a problem for a public accessible page.
for the following code:

<a href="edit" tal:omit-tag="python: user.getId()!=username">
username
</a>
......(other stuffs)

what I want to do is simple: the hyperlink only be invokable by user-self.
My problem is that whenever  "user"  was requested in a tal expression,
this page return nothing(empty) which causes the browser asks me to save
this dummy page.
As well as requesting  "request.AUTHENTICATED_USER"  got the same problem.

Why? I am using Zope 2.4.3 in a RedHat 7.1 box.
Any suggestion is appreciated.
Iap, Singuan