MySQL“多对多”关系错误

我有三张“多对多”关系的表。

表A:

CREATE TABLE IF NOT EXISTS tabA(
tabAcolA mediumint unsigned not null auto_increment,
tabAcolB text not null,
tabAcolC text null,
PRIMARY KEY keyA (tabAcolA)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

表B:

CREATE TABLE IF NOT EXISTS tabB(
tabBcolA mediumint unsigned not null auto_increment,
tabBcolB text not null,
PRIMARY KEY keyB (tabBcolA)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

表C(AB接头):

CREATE TABLE IF NOT EXISTS tabC(
    tabCcolA int unsigned not null,
    tabCcolB int unsigned not null,
    PRIMARY KEY keyC (tabCcolA,tabCcolB),
    CONSTRAINT FkeyA FOREIGN KEY (tabCcolA) 
    REFERENCES tabA(tabAcolA) ON DELETE RESTRICT ON UPDATE RESTRICT,
    CONSTRAINT FkeyB FOREIGN KEY (tabCcolB) 
    REFERENCES tabB(tabBcolA) ON DELETE RESTRICT ON UPDATE RESTRICT
    ) ENGINE=InnoDB

它返回一个错误:

1215 Cannot add foreign key constraint

怎么了?