Getting Tags,Categories by Post Type Wordpress/get_terms shows empty array

Recently I faced an Issue of getting tags of a Custom Postype created by Me and I solved it as follows
I registered a taxonomy for the same as follows

For Tags
register_taxonomy(ad_tag,
            array('my_cust_posttype'),
            array('hierarchical' => false,
                  'labels' => array(
                        'name' => __( 'Ad Tags', 'appthemes'),
                        'singular_name' => __( 'Ad Tag', 'appthemes'),
                        'search_items' =>  __( 'Search Ad Tags', 'appthemes'),
                        'all_items' => __( 'All Ad Tags', 'appthemes'),
                        'parent_item' => __( 'Parent Ad Tag', 'appthemes'),
                        'parent_item_colon' => __( 'Parent Ad Tag:', 'appthemes'),
                        'edit_item' => __( 'Edit Ad Tag', 'appthemes'),
                        'update_item' => __( 'Update Ad Tag', 'appthemes'),
                        'add_new_item' => __( 'Add New Ad Tag', 'appthemes'),
                        'new_item_name' => __( 'New Ad Tag Name', 'appthemes')
                    ),
                    'show_ui' => true,
                    'query_var' => true,
                    'update_count_callback' => '_update_post_term_count',
                    'rewrite' => array( 'slug' => $tag_tax_base_url, 'with_front' => false ),
            )
    );

For Categories
    register_taxonomy( APP_TAX_CAT,
            array(APP_POST_TYPE),
            array('hierarchical' => true,
                  'labels' => array(
                        'name' => __( 'Ad Categories', 'appthemes'),
                        'singular_name' => __( 'Ad Category', 'appthemes'),
                        'search_items' =>  __( 'Search Ad Categories', 'appthemes'),
                        'all_items' => __( 'All Ad Categories', 'appthemes'),
                        'parent_item' => __( 'Parent Ad Category', 'appthemes'),
                        'parent_item_colon' => __( 'Parent Ad Category:', 'appthemes'),
                        'edit_item' => __( 'Edit Ad Category', 'appthemes'),
                        'update_item' => __( 'Update Ad Category', 'appthemes'),
                        'add_new_item' => __( 'Add New Ad Category', 'appthemes'),
                        'new_item_name' => __( 'New Ad Category Name', 'appthemes')
                    ),
                    'show_ui' => true,
                    'query_var' => true,
                    'update_count_callback' => '_update_post_term_count',
                    'rewrite' => array( 'slug' => $cat_tax_base_url, 'with_front' => false, 'hierarchical' => true ),
            )
    );

And Then I retrieved Tags for my custom post type as
$mylinks_tags = get_terms('ad_tag','orderby=name&hide_empty=0');

remember the parameter hide_empty if you are starting afresh you will get an empty array if this is not set to 0 by default this is 1 which means all empty terms will be hidden.Scratched my head for a couple of hours to figure this out