[Zope] Recursive Calling the same DTML Method is not validated?

[iap@y2fun.com]

Suppose we have a DTML Method named "A"
---------------------------------------------------------
<dtml-unless "REQUEST.has_key('aa')">
    <dtml-call "REQUEST.set('aa',0)">
</dtml-unless>

<dtml-call "REQUEST.set('aa',REQUEST['aa']+1)">
<dtml-var "REQUEST['aa']">

<dtml-if "REQUEST['aa']>10">
   <dtml-return "''"><dtml-comment>Stop here</dtml-comment>
<dtml-else>
   <dtml-var A><dtml-comment>Go deeper</dtml-comment>
</dtml-if>
---------------------------------------------------------
Excute it, then got the result:

Error Type: KeyError
Error Value: validate
---------------------------------------------------------
This is a sample method abstracted from my another more complex method
which works in the previous version of Zope.
I know that there are something changed in the SecurityManager.
But I have no idea how to make this sort of recursive method works again.
Any suggestion?
Thanks.
Iap, Singuan