Error Code: 1822. Failed to add the foreign key constraint. Missing index for constraint 'employees_ibfk_1' in the referenced table 'departments' 0.000 sec

Hello everyone!
I was trying to create those 2 table on my database, and this Error Message keeps showing, I don’t know how to solve it. Could someone help me?

CREATE TABLE departments (
department_id DECIMAL(4,0) NOT NULL DEFAULT 0 PRIMARY KEY,
department_name VARCHAR(30) NOT NULL,
manager_id DECIMAL(6,0) NOT NULL DEFAULT 0,
location_id DECIMAL(4,0) NOT NULL,
UNIQUE (department_id, manager_id)
);
CREATE TABLE employees (
employee_id VARCHAR(10) UNIQUE NOT NULL PRIMARY KEY,
first_name VARCHAR(20) NOT NULL,
last_name VARCHAR(30) NOT NULL,
email VARCHAR(70),
phone_number VARCHAR(15),
hire_date DATE NOT NULL,
job_id VARCHAR(10) NOT NULL,
salary DECIMAL(8,2) NOT NULL,
comission DECIMAL(2,2),
manager_id DECIMAL(6,0) NOT NULL DEFAULT 0,
department_id DECIMAL(4,0) NOT NULL DEFAULT 0,
FOREIGN KEY (manager_id) REFERENCES departments(manager_id),
FOREIGN KEY (department_id) REFERENCES departments(department_id)
);

Error Code: 1822. Failed to add the foreign key constraint. Missing index for constraint ‘employees_ibfk_1’ in the referenced table ‘departments’

Thank you very much in advance for your help.