Exception: Returned Type Mismatch Error

[devansing]

I'm taking an online course and have to submit the code through a 3rd party grading system. I have all of the scripts working as intended locally but when I submit it to the online grader, I'm getting some confusing errors –– the grader is running my functions with its own inputs.

Here's one example:

import datetime

def days_in_month(year, month):
    """
    Inputs:
      year  - an integer between datetime.MINYEAR and datetime.MAXYEAR
              representing the year
      month - an integer between 1 and 12 representing the month

    Returns:
      The number of days in the input month.
    """

    if (month < 1 or month > 12) or (year < 1 or year > 9999):
        return 0

    first_of_month = datetime.date(year, month, 1)

    if month == 12:
        first_of_next_month = datetime.date(year + 1, 1, 1)

    else:
        first_of_next_month = datetime.date(year, month + 1, 1)

        difference = first_of_next_month - first_of_month

        return difference.days

Error:
days_in_month(12, 12) expected 31 but received "(Exception: Returned Type Mismatch) Expected type 'int' but returned type 'NoneType'."

Appreciate any help -- thanks!