Shortcode error

Hi!
I’m trying to add some blocks via shortcodes.

First, I’ve added to category description (all is OK):

add_filter( 'term_description', 'filter_function_name_386', 10, 4 );
function filter_function_name_386( $value, $term_id, $taxonomy, $context ){
	// filter...

	return $value .= do_shortcode( '[the_ad id="1681"]' ); 
}

After that I tried to do the same for tags, but it fails:

add_filter( 'post_tag_description', 'filter_function_name_385', 10, 4 );
function filter_function_name_385( $value){
	// filter...

        //shortcode returns an empty string here
	$aaa = do_shortcode( '[the_ad id="1681"]');

	return $value. $aaa . 'ttt';
}

So do_shortcode returns an empty string here, how can I fix it? Thanks