How to set custom post terms for new post in P2?

Hi, everybody!
I’m using wordpress 3.0 beta1 and P2 v1.1.3.
I tried add custom post terms for new post in P2.
I has a custom taxonomy called “letter”, I add code below in p2/inc/ajax.php(line 176, before “echo $post_id ? $post_id : ‘0’;”):

wp_set_post_terms( $post_id, ‘A,B’, ‘letter’);

this didn’t work.

but wp_set_post_tags($post_id,’A,B’) is work well.
wp_set_post_tags() function in wp-includes/post.php, here it’s code.

function wp_set_post_tags( $post_id = 0, $tags = ”, $append = false ) {
return wp_set_post_terms( $post_id, $tags, ‘post_tag’, $append);
}

So, I creat a new function:

function wp_set_post_letters( $post_id = 0, $tags = ”, $append = false ) {
return wp_set_post_terms( $post_id, $tags, ‘letter’, $append);
}

and I use wp_set_post_letters($post_id,’A,B’), it is still didn’t work.

Forgive me my bad english, Anyone has a solution for this?