Inherit taxonomy using drupal_execute

I don't really know if it's an Aggregator Node bug or not.
Aggregator Node uses drupal_execute to create a node.
For inheriting taxonomy, here is the code piece:

if (is_object($parent_node) && !empty($parent_node->taxonomy)) {
    $node['taxonomy'] = $parent_node->taxonomy;
    foreach ($node['taxonomy'] as $k => $v) {
    	if (is_array($v) && $k != 'tags') {
    	  foreach ($v as $t1 => $t2) {
    	    $node['taxonomy'][] = taxonomy_get_term($t2);
    	  }
    	}
    }
  }

This code works well with both free-tagging and fixed vocabularies. But while creating the item nodes, php notices appears:

# notice: Trying to get property of non-object in /var/www/html/drupal7/modules/taxonomy/taxonomy.module on line 1282.
# notice: Trying to get property of non-object in /var/www/html/drupal7/modules/taxonomy/taxonomy.module on line 1282.

Should Aggregator Node populates $node['taxonomy'] in a different way or it's a bug in taxonomy.module?