How to keep multiple values selected dropdown in magento addField (multiselect)

$filterData = $this->helper('adminhtml')->prepareFilterString($this->getRequest()->getParam('filter', false));
$fieldset->addField('warehouse', 'multiselect', array(
            'label' => Mage::helper('namespace_reports')->__('Warehouse'),
            'title' => Mage::helper('namespace_reports')->__('Select multiple warehouse'),
            'name' => 'warehouse',
            'values' => $this->_getWarehouseArr(),
            'value' => isset($filterData['warehouse'][0]) ? explode(',', $filterData['warehouse'][0]) : array(),
            'style' => 'height:43px'
        ));