Comment Errors

I just upgraded the new WordPress and many of my commenters report errors when they leave a comment. Here’s what the error said for one of them… any suggestions?

Warning: in_array() [function.in-array]: Wrong datatype for second argument in /home/friendl5/public_html/wp-content/plugins/subscribe-to-comments.php on line
438
Warning: Cannot modify header information – headers already sent by (output started at /home/friendl5/public_html/wp-content/plugins/subscribe-to-comments.php:438)
in /home/friendl5/public_html/wp-content/plugins/wp-ajax-edit-comments/wp-ajax-edit-comments.php on line 705
Warning: Cannot modify header information – headers already sent by (output started at /home/friendl5/public_html/wp-content/plugins/subscribe-to-comments.php:438)
in /home/friendl5/public_html/wp-comments-post.php on line 67
Warning: Cannot modify header information – headers already sent by (output started at /home/friendl5/public_html/wp-content/plugins/subscribe-to-comments.php:438)
in /home/friendl5/public_html/wp-comments-post.php on line 68
Warning: Cannot modify header information – headers already sent by (output started at /home/friendl5/public_html/wp-content/plugins/subscribe-to-comments.php:438)
in /home/friendl5/public_html/wp-comments-post.php on line 69
Warning: Cannot modify header information – headers already sent by (output started at /home/friendl5/public_html/wp-content/plugins/subscribe-to-comments.php:438)
in /home/friendl5/public_html/wp-includes/pluggable.php on line 331