[Plugin: Posts By Tag] Notice on post update/publish, causing error

When I update an existing blog post or publish a new one, I get this Notice, which causes a headers already sent error. It also appears at the top of the page when trying to create a new post.

Notice: Undefined index: posts_by_tag_noncename in [dir]/wp-content/plugins/posts-by-tag/posts-by-tag.php on line 156

http://ww.wp.xz.cn/extend/plugins/posts-by-tag/