Fatal error when enabling node follow-ups

When enabling node follow-ups for a user it becomes impossible to post a comment on that user's nodes.

Repeatable: Always

Steps to repeat:
1) Enable option to receive follow-ups for nodes for a user
2) Post a comment for that user's node.

Expected Results:
Comment gets published.

Actual Results:

Fatal error: Unsupported operand types in /path/to/drupal/includes/common.inc on line 1369

System info:

Drupal version...: 6.8
PHP version......: 5.2.6
MySQL database...: 5.0.68