约束的SQL语法错误

Question

约束和表格不对，不确定在哪里。第一个约束下的红线和SQL脚本中的vehicle_code：

create table hires (
    vehicle_code_id varchar(25) primary key,
    customer_id int not null,
    employee_id int not null,
    hired_from_date datetime not null,
    hired_to_date datetime not null,
    bond smallmoney not null,

    constraint hires_pk primary key (hired_from_date),
    constraint vehicle_code_fk foreign key (vehicle_code) references hires (vehicle_code_id),
    constraint customer_fk foreign Key (customer_id) references customers (customer_id),
    constraint employee_fk foreign key (employee_id) references employees (employee_id)
)