def decode_tag(stream):
    """Decode a tag value from a serialized stream.

    :param stream: Source data stream
    :type stream: io.BytesIO
    :returns: Decoded tag
    :rtype: bytes
    """
    (reserved, tag) = unpack_value(">cc", stream)

    if reserved != b"\x00":
        raise DeserializationError("Invalid tag: reserved byte is not null")

    return tag