Threads. I inspected the importer script to see what it was doing and then did some investigation with MySQL:

Code
mysql> USE old_database_name;
mysql> SELECT * FROM w3t_Banned;
+------------+
| B_Hostname |
+------------+
| NULL       |
+------------+
1 row in set (0.00 sec)

I deleted that row and then resumed the import by going to http://example.com/path_to_forums/importers/threads_import.php?ubb=extras

The import then completed. I have no idea how/why that NULL value got into the table.