PHP errors when accessing feeds for activity

Drupal 6.10

When accessing feeds for either site or user activity streams, we're seeing two errors appearing in our dblog, though the feeds appear intact and accurate.

Object of class stdClass could not be converted to string in /var/www/oursite/includes/bootstrap.inc on line 768.
preg_match() expects parameter 2 to be string, object given in /var/www/oursite/includes/bootstrap.inc on line 771.

The referenced lines in bootstrap.inc are the drupal_validate_utf8 function

 767 function drupal_validate_utf8($text) {
 768   if (strlen($text) == 0) {
 769     return TRUE;
 770   }
 771   return (preg_match('/^./us', $text) == 1);
 772 }

Any thoughts on how to resolve the problem?