Comments problem after upgrade to 2.1

When I try to post comments on my site after upgrading I get the following error:

Warning: in_array() [function.in-array]: Wrong datatype for second argument in /home/.dickie/——-my directory——/wp-content/plugins/subscribe-to-comments.php on line 434

Warning: Cannot modify header information – headers already sent by (output started at /home/.dickie/——-my directory——/wp-content/plugins/subscribe-to-comments.php:434) in /home/.dickie/——-my directory——/wp-includes/pluggable.php on line 275

The comments do appear on my site but I would like to solve this issue if anyone has any ideas??

Thanks Pamela