SQL错误1215无法添加外键

Question

     create table branch(
     brname char(20) not null,
     city char(20) not null,
     assests float(8,2) not null,
     locid int not null,
     coname char(20) not null,
     primary key(coname,brname)
     )engine=innodb;
     alter table branch
     add constraint fk_branch_city foreign key(coname,locid)
     references city(coname,locid);

     create table room(
     brname char(20) not null,
     roomnb int not null,
     costofroom float(8,2) not null,
     roomsize int not null,
     roomfloor int not null,
     primary key(roomnb,brname)
     )engine=innodb;
     alter table room
     add constraint fk_room_branch foreign key(brname)
     references branch(brname);

我得到错误：sql执行错误1215，无法添加外部约束键（alter table room until branch（brname）;正在突出显示）。 所以我只是不明白我该如何解决这个问题。