def normalize_prefix(path):
    """Add trailing slash to prefix if it is not present
    >>> normalize_prefix("somepath")
    'somepath/'
    >>> normalize_prefix("somepath/")
    'somepath/'
    """
    if path is None or path is '' or path is '/':
        return None
    elif path.endswith('/'):
        return path
    else:
        return path + '/'