"Add block" error

When try adding a new block, I receive following error (paths sanitized):

user error: Duplicate entry '' for key 3
query: INSERT INTO boxes (title, body, info, format) VALUES  ('New title', '<h1>Hi!</h1>', '', 1) in /home/path/to/drupal/includes/database.mysql.inc on line 66.

warning: Cannot modify header information - headers already sent by (output started at /home/path/to/drupal/includes/common.inc:384) in /home/path/to/drupal/includes/common.inc on line 192.

Previously, I added 2 new blocks with appear in the "boxes" table, so it worked at one point. I don't understand how this can be duplicate entry in the error msg. reference to "key 3".

I did a check on the database (using database module), which declares the table is fine.

Any thoughts?

Best,
Brent
http://www.wanderings.net/books/