Appearantly $delta is not always a number
I tried to use this php code to show the feed-content of an aggregator feed on a page. It turned up to work out this way:
<?php
$block = module_invoke('aggregator', 'block', 'view', 'feed-1');
print $block['content'];
?>So $delta here is supposed to be of the form 'feed-x' where x stands for the number of the feed.