MySQL Error 1215: cannot add foreign key constraint

CREATE TABLE Professor(
	ra INT  UNSIGNED NOT NULL 
    ,apelido VARCHAR(30) NOT NULL
    ,nome VARCHAR(120) NOT NULL
    ,email VARCHAR(80) NOT NULL
    ,celular CHAR(11) NOT NULL
    ,CONSTRAINT pkra PRIMARY KEY (ra)
    ,CONSTRAINT uqapelido UNIQUE (apelido) 
    )ENGINE=InnODB;

CREATE TABLE Curso(
	sigla VARCHAR(5) NOT NULL 
    ,nome VARCHAR(50) NOT NULL
    ,CONSTRAINT pksigla PRIMARY KEY (sigla)
    ,CONSTRAINT uqnome UNIQUE (nome)
    )ENGINE=InnODB;
    
CREATE TABLE GradeCurricular(
	sigla_curso VARCHAR(5) NOT NULL
    ,ano SMALLINT UNSIGNED NOT NULL
    ,semestre CHAR(1) NOT NULL
    ,CONSTRAINT fksigla_curso FOREIGN KEY (sigla_curso) REFERENCES Curso (sigla) 
    ,PRIMARY KEY (sigla_curso,ano,semestre)
)ENGINE=InnODB;
	
CREATE TABLE Periodo(
	sigla_curso VARCHAR(5) NOT NULL
    ,ano_grade SMALLINT UNSIGNED NOT NULL
    ,semestre_grade CHAR (1) NOT NULL
    ,numero TINYINT UNSIGNED NOT NULL
    ,CONSTRAINT fk_sigla_curso FOREIGN KEY (sigla_curso) REFERENCES GradeCurricular (sigla_curso)
    ,CONSTRAINT fk_ano_grade FOREIGN KEY (ano_grade) REFERENCES GradeCurricular (ano)
    ,CONSTRAINT fk_semestre_grade FOREIGN KEY (semestre_grade) REFERENCES GradeCurricular (semestre)
    ,PRIMARY KEY (sigla_curso,ano_grade,semestre_grade,numero)
)ENGINE=InnODB;