locale.currency problem?

I’m running the following script (from Murach’s Python Programming Ch01), using Python 3.6.1 on OS X Sierra.  It runs fine when invoked from the Terminal, and when run inside IDLE, but fails when run within PyCharm Edu 2.0.4, with “ValueError: Currency formatting is not possible using the 'C' locale.”  Why the different behaviour?

I’m new at all of this so if you need further information, please ask.

#!/usr/bin/env python3

import locale

# set the locale for use in currency formatting
result = locale.setlocale(locale.LC_ALL, '')
if result == 'C':
    locale.setlocale(locale.LC_ALL, 'en_US')

# display a welcome message
print("Welcome to the Future Value Calculator")
print()

choice = "y"
while choice.lower() == "y":

    # get input from the user
    monthly_investment = float(input("Enter monthly investment:\t"))
    yearly_interest_rate = float(input("Enter yearly interest rate:\t"))
    years = int(input("Enter number of years:\t\t"))

    # convert yearly values to monthly values
    monthly_interest_rate = yearly_interest_rate / 12 / 100
    months = years * 12

    # calculate the future value
    future_value = 0
    for i in range(months):
        future_value = future_value + monthly_investment
        monthly_interest_amount = future_value * monthly_interest_rate
        future_value = future_value + monthly_interest_amount

    # format and display the result
    print("Future value:\t\t\t%s" % locale.currency(
        future_value, grouping=True))
    print()

    # see if the user wants to continue
    choice = input("Continue? (y/n): ")
    print()

print("Bye!")