Foreign key error

Question

hi my code is giving error #1005 - Can not create table foo.aula (error no. 150 "Foreign key Constraint is incorrectly Formed"

DROP TABLE IF EXISTS `aula`;
CREATE TABLE IF NOT EXISTS `aula` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `turma_id` int(11) NOT NULL,
  `professor_id` int(11) NOT NULL,
  `aluno_id` int(11) NOT NULL,
  `valor` float NOT NULL,
  `data` varchar(45) NOT NULL,
  `horarioInicio` varchar(45) NOT NULL,
  `horarioTermino` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_aula_aluno_idx` (`aluno_id`),
  KEY `fk_aula_professor_idx` (`professor_id`),
  KEY `fk_aula_turma_idx` (`turma_id`),
  CONSTRAINT   `fk_aula_aluno`  FOREIGN KEY (`aluno_id`)  REFERENCES `aluno` (`id`)  ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT `fk_aula_professor` FOREIGN KEY (`professor_id`) REFERENCES `professor` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT `fk_aula_turma` FOREIGN KEY (`turma_id`) REFERENCES `turma` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=latin1;