Max Key Length Error

[Brian Rivet]

Sorry, I'm a little new at this, here's the statement:
 
CREATE TABLE `TblUsers` (`UserId` INT(5) UNSIGNED DEFAULT NULL NOT NULL
AUTO_INCREMENT, `FirstName` VARCHAR(255) DEFAULT 'unknown' NOT NULL,
`LastName` VARCHAR(255) DEFAULT 'unknown' NOT NULL, `UserName`
VARCHAR(255) DEFAULT 'unknown' NOT NULL, `UserPass` VARCHAR(255) DEFAULT
'unknown' NOT NULL , PRIMARY KEY (`UserId`), INDEX (`UserId`,
`LastName`, `UserName`), UNIQUE (`UserId`, `UserName`))
 
It says the max key length is 500, I'm not sure what it's referring to,
so I don't know what to fix.
 
Thanks for any help you can give me.
 
Brian Rivet