MYSQL Query - make field greater than all the others

Mysql Max

[HeadBut]

Whats wrong with this:

Update `items` SET Adid = (SELECT MAX(Adid) From `items` WHERE 1 = 1)+1 WHERE ActNum = 1000 ;

Just want to make all the items for account 1000 have an Adid that is greater than all the others.

Thanks