Error Code: 1048. Column 'manager_no' cannot be null.

I tried to compare my written queries to the given answer but I cannot find the difference. Below is my answer. Kindly assist. Thank you.

CREATE TABLE emp_manager (
    emp_no INT(11) NOT NULL,
    dept_no CHAR(4) NULL,
    manager_no INT(11) NOT NULL
);

Insert INTO emp_manager SELECT
U.* FROM (SELECT A.* FROM
(SELECT
e.emp_no AS employee_ID,
    MIN(de.dept_no) AS department_code,
(SELECT emp_no FROM dept_manager WHERE emp_no = 110022) AS manager_ID
FROM employees e
JOIN dept_emp de ON e.emp_no = de.emp_no
WHERE e.emp_no <= 10020
GROUP BY e.emp_no
ORDER BY e.emp_no)AS A
UNION
SELECT B.* FROM
(SELECT
e.emp_no AS employee_ID,
    MIN(de.dept_no) AS department_code,
(SELECT emp_no FROM dept_manager WHERE emp_no = 110039) AS manager_ID
FROM employees e
JOIN dept_emp de ON e.emp_no = de.emp_no
WHERE e.emp_no BETWEEN 10021 AND 10040
GROUP BY e.emp_no
ORDER BY e.emp_no)AS B
UNION
SELECT C.* FROM
(SELECT
e.emp_no AS employee_ID,
    MIN(de.dept_no) AS department_code,
(SELECT emp_no FROM dept_manager WHERE emp_no = 110039) AS manager_ID
FROM employees e
JOIN dept_emp de ON e.emp_no = de.emp_no
WHERE e.emp_no = 110022
GROUP BY e.emp_no)AS C
UNION
SELECT D.* FROM
(SELECT
e.emp_no AS employee_ID,
    MIN(de.dept_no) AS department_code,
(SELECT emp_no FROM dept_manager WHERE emp_no = 110021) AS manager_ID
FROM employees e
JOIN dept_emp de ON e.emp_no = de.emp_no
WHERE e.emp_no = 110039
GROUP BY e.emp_no)AS D)AS U;