Custom expanded Low stock report Search shows no results when filtering one specific column (“Parent SKU”)

I expanded the Low stock report so that when a simple product is a component of a configurable, the configurable sku will appear as “Parent SKU” to the right of the simple product’s sku and name. The alias “parentsku” is being used for the “sku” field in the ‘catalog/product’ to avoid conflict with “sku” in resource model ‘reports/product_lowstock_collection’

Displays properly on initial load of report, but filtering on “Parent SKU” yields empty result even if populated in some records. All other columns filter properly.

From my Grid.php file…

$collection = Mage::getResourceModel(‘reports/product_lowstock_collection’)
->addAttributeToSelect(‘*’)
->setStoreId($storeId)
->filterByIsQtyProductTypes()
->joinInventoryItem(‘qty’)
->setOrder(‘qty’, Varien_Data_Collection::SORT_ORDER_ASC);

$collection->getSelect()->joinLeft(array(‘link_table’ => $collection->getTable(‘catalog/product_super_link’)),
‘link_table.product_id = e.entity_id’,
array(‘product_id’)
);

$collection->getSelect()->joinLeft(array(‘par_table’ => $collection->getTable(‘catalog/product’)),
‘link_table.parent_id = par_table.entity_id’,
array(‘sku as parentsku’)
);

/////////////////////////////

protected function _prepareColumns()
{
    $this->addColumn('sku', array(
        'header'    =>Mage::helper('reports')->__('Product SKU'),
        'sortable'  =>false,
        'index'     =>'sku'
    ));

    $this->addColumn('name', array(
        'header'    =>Mage::helper('reports')->__('Product Name'),
        'sortable'  =>false,
        'index'     =>'name'
    ));

    $this->addColumn('parentsku', array(
        'header'    =>Mage::helper('reports')->__('Parent SKU'),
        'sortable'  =>false,
        'index'     =>'parentsku'
    ));

    $this->addColumn('qty', array(
        'header'    =>Mage::helper('reports')->__('Stock Qty'),
        'width'     =>'215px',
        'align'     =>'right',
        'sortable'  =>false,
        'filter'    =>'adminhtml/widget_grid_column_filter_range',
        'index'     =>'qty',
        'type'      =>'number'
    ));
}