How can i get a random string in MySQL?

I'm trying to get a random string in phpmyadmin using a function. I have the followin code:

CREATE FUNCTION randomPassword()
RETURNS varchar(128)
BEGIN

SET @chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
SET @charLen = length(@chars);

SET @randomPassword = '';

WHILE length(@randomPassword) < 12
    SET @randomPassword = concat(@randomPassword, substring(@chars,CEILING(RAND() * @charLen),1));
END WHILE;

RETURN @randomPassword ;
END;

Now i get the error:

1064 - 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 '' at line 5

does anyone know ho it has to be?