MySQL Error #1071 - Specified key was too long; max key length is 767 bytes

When I executed the following command: ALTER TABLE `mytable` ADD UNIQUE ( `column1` , `column2` ); I got this error message: #1071 - Specified key was too long; max key length is 767 bytes Information about column1 and column2: column1 varchar(20) utf8_general_ci column2 varchar(500) utf8_general_ci I think `varchar(20)` only requires 21 bytes while `varchar(500)` only requires 501 bytes. So the total bytes are 522, less than 767. So why did I get the error message? #1071 - Specified key was too long; max key length is 767 bytes