virtuemart_category_id always return 0

[teodorus]

Hi Expert,

I'm using virtuemart 2.5, my site is pintarbelanja.com

in templates/[my template]/html/mod_virtuemart_search/default.php
there is a code <form id="ui_element" class="default-search form-search" action="<?php echo JRoute::_('index.php?option=com_virtuemart&view=category&search=true&limitstart=0&virtuemart_category_id='.$category_id ); ?>" method="get">

Why whenever i give value $category_id = 1 it always parse virtuemart_category_id = 0.
I already debug it in class VirtuemartControllerCategory, VirtuemartViewCategory, and also the models, but virtuemart_category_id is always 0.
I need to parse category id that choosen by user to filter the search.

Need your help please, I already spent 3 days to figure it out but I stack and can't find the answer.

Thanks.