Resolved by switching to Python 2.

Python test code is failing on "Testing for 120" with the following trace:

Traceback:
   in 
   in sol_checker
   in simple_sol
   in simple_sol
TypeError: list indices must be integers, not float

My actual solution returns the expected string when tested locally with input 120; there are no floats involved.

Double-checked the problem by submitting this:

def int_to_english(n):
    if n == 1:
        return "one"
    elif n == 19:
        return "nineteen"
    elif n == 120:
        return "one hundred twenty"

The failure is the same.