SQL error when using the Edit Databases facility.

Using the Edit Databases facility:

When trying to edit a table field length I always get this error:

Failed to save field : SQL alter table `adverts` modify `caption` varchar(200) not null default none failed : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'none' at line 1

Closed (fixed)