Q&A: Plugin breaking at bottom of page

Hi - I'm getting an error at the bottom of the Q & A page where people can subscribe.


Warning: in_array() [function.in-array]: Wrong datatype for second argument in /home/linweb09/f/fitness-saver.com/user/htdocs/wp-content/plugins/qa/core/subscriptions.php on line 20

I checked that line number, and I see the following:

$subscribers = get_post_meta( $id, '_sub' );
$subscribed = in_array( get_current_user_id(), $subscribers ); <-- error coming from 2nd param here

I'm not sure why this error is happening - can you help please?