def render_pep440(vcs):
    """Convert git release tag into a form that is PEP440 compliant."""

    if vcs is None:
        return None

    tags = vcs.split('-')

    # Bare version number
    if len(tags) == 1:
        return tags[0]
    else:
        return tags[0] + '+' + '.'.join(tags[1:])