db_change_field creating bad sql

Hello,
Is there something wrong with this command to change a field called 'quantity' from an integer to a float?

db_change_field($ret, 'cc_transactions', 'quantity', 'quantity', array(
  'description' =>'number of units transacted (float)',
  'type'=>'real',
  'not null' => TRUE,
);

I'm getting this error message:
check the manual that corresponds to your MySQL server version for the right syntax to use near 'NOT NULL' at line 1 query:
ALTER TABLE cc_transactions
CHANGE quantity `quantity` NOT NULL
in /home/localcur/public_html/includes/database.mysql-common.inc on line 520.

Problem doesn't seem to occur when converting the other way.
Or is my command, or is there something wrong with the db layer?
Thanks