Issue1724

def join(a, *p):
    """Join two or more pathname components, inserting '/' as needed"""
    path = a
    for b in p:
        if b is None:            <<<<<<< below two lines is added by me
            return path
        if b.startswith('/'):
            path = b
        elif path == '' or path.endswith('/'):
            path +=  b
        else:
            path += '/' + b
    return path

This is just how os.path.join API works, you're not supposed to pass it None values.

Also, Jython generally doesn't change behavior like this without CPython doing it first.