Database creation fails due to primary key length limit on MySQL

[juliusknorr]

  [Doctrine\DBAL\Exception\DriverException]                                                                                            
  An exception occurred while executing 'CREATE TABLE `oc_spreedme_room_participants` (`userId` VARCHAR(256) NOT NULL, `roomId` VARCH  
  AR(256) NOT NULL, `lastPing` INT NOT NULL, PRIMARY KEY(`userId`)) DEFAULT CHARACTER SET utf8 COLLATE utf8_bin ENGINE = InnoDB':      
  SQLSTATE[42000]: Syntax error or access violation: 1071 Specified key was too long; max key length is 767 bytes                      

  [Doctrine\DBAL\Driver\PDOException]                                                                              
  SQLSTATE[42000]: Syntax error or access violation: 1071 Specified key was too long; max key length is 767 bytes  

  [PDOException]                                                                                                   
  SQLSTATE[42000]: Syntax error or access violation: 1071 Specified key was too long; max key length is 767 bytes