Permalink problem: “uncategorized” has been added.

I recently upgraded my blog from v2.5.1 to v2.7, and I’m noticing that my permalink structure has changed.

In the previous version, a typical URL was:

http://www.sitename.com/blog/a-typical-blog-post

and now, ‘uncategorized’ has been introduced:

http://www.sitename.com/uncategorized/blog/a-typical-blog-post

[Sigh]

Must I do a manual edit of the DB thru phpmyadmin to correct this? A quick glance of the permalink structure shows that nothing has been altered.