How to fix error with json library on heroku?

Question

Python

import json
json_string = """ { "name":"John", "age":30, "car":null } """
parsed_string = json.loads(json_string)

This code throws an error

AttributeError: module 'json' has no attribute 'loads'

The whole interest of the question is why this code works fine on the local computer, but when uploaded to the server, it starts to give errors? And how to fix it?