Warning on Incorrect table definition

I got the warning after just enabling the module in drupal 6rc1

user warning: Incorrect table definition; there can be only one auto column and it must be defined as a key query: CREATE TABLE drupal6knurl ( `tid` INT unsigned NOT NULL auto_increment, `uid` INT unsigned NOT NULL DEFAULT 0, `short_url` VARCHAR(255) NOT NULL DEFAULT '', `link` TEXT NOT NULL DEFAULT '' ) /*!40100 DEFAULT CHARACTER SET UTF8 */ in /home/bakr78/public_html/cms/includes/database.inc on line 487.