caching not working

I have a list of id’s I want cached so I don’t have to built it oall the time but for some reason it never caches. Any idea what could be wrong?

$top_parents_ids = wp_cache_get( 'cached_top_parent_ids' );
if ( false == $top_parents_ids ) {
	echo('not cached');
	$top_parents = array('South America','North America','Europe','Oceania','Africa');
	$top_parents_ids = array();
	foreach($top_parents as $top_parent){
		if ($term = term_exists($top_parent,'photo_gallery'))
			array_push($top_parents_ids,$term['term_id']);
	}
	wp_cache_set('cached_top_parent_ids', $top_parents_ids);
}