About iw_iterator

System

Hi,
I have one question about templating file, I want to list all items and print br tag each of three items.
My code is as below:

<iw_iterate var="x" list="dcr.CatInto" iteration="current_number">
<iw_value name='x.CatName'/>
<iw_if expr='{$current_iteration_number%4 == 3 }'>
<iw_then>
<br>
</iw_then>
</iw_if>
</iw_iterate>

All looks good, but i find that the tag <iw_if> doesn't work, even if my code is changed to <iw_if expr='{$current_num == 0}'>,
the expr still regards the result as true....
I don't know what's wrong with this. could anyone tell me?
Thanks a lot!!!!!!!!!!!!!