Python code to count business days in ArcGIS field calculator gives ERROR 999999?

Question

I'm trying to field calculate using a python script the number of business days between two given dates. Right now the dates are hard coded, but once I get the script working I will pass dates from two other fields to it. The script works fine in IDLE, giving me the correct number of business days, but when I put it in the field calculator it errors out:

ERROR 999999: Error executing function

The field is not nullable.

The field is set as "Short" with Precision at 5.

Pre-Logic Script Code:

import datetime as dt
import numpy as np

def workdays():
    start = dt.date(2018, 1, 31)
    end = dt.date(2018, 2, 12)
    days = np.busday_count(start, end)
    return days

Field=

workdays()

Answer 1

Your function is returning data type numpy.int64 which is probably not compatible with field type Short. Try return int(days)