Having Trouble With get_category_by_slug In a Custom Function

function ribbon_list_cats( $parent_cat_slug, $echo = true )
{
    $parent_category = get_category_by_slug( $parent_cat_slug );
    if ( $parent_category->count > 0 ) 
    {
        $output="<ul>";
        $output .= wp_list_categories('title_li=&child_of=".$parent_category->term_id."&hide_empty=0&show_option_none=&echo=0');
        $output .= '</ul>';
    }
    // Debug: uncomment the following line
    // echo '<pre>'; var_dump( $output ); echo '</pre>';

    if ( $echo === false )
        return $output;

    return print $output;
}

// Call it like this:
ribbon_list_cats( 'uncategorized' );