Help me in solving SQW33 problem

My issue

Not able to solve the problem

My code

/* Write a query to perform the following
- Join the tables - 'product_catalog' and division_product_sales' 
- Output product_id, units_sold and month from the joined table
- Remember the condition - we need to identify the worst performing month for each product_id in terms of units sold
- Order by product_id */
WITH ranked_sales AS (
  SELECT
    pc.product_id,
    dps.units_sold,
    dps.month,
    RANK() OVER (PARTITION BY pc.product_id ORDER BY dps.units_sold ASC) AS rank_num
  FROM
    product_catalog pc
  JOIN
    division_product_sales dps ON pc.product_id = dps.product_id
)
SELECT
  rs.product_id,
  rs.units_sold,
  rs.month
FROM
  ranked_sales rs
WHERE
  rs.rank_num = 1
ORDER BY
  rs.product_id;

Learning course: Learn Data Analytics using SQL and Python
Problem Link: Practice Problem in - CodeChef