def construct_null_bdev(client, num_blocks, block_size, name, uuid=None):
    """Construct a null block device.

    Args:
        num_blocks: size of block device in blocks
        block_size: block size of device; must be a power of 2 and at least 512
        name: name of block device
        uuid: UUID of block device (optional)

    Returns:
        Name of created block device.
    """
    params = {'name': name, 'num_blocks': num_blocks,
              'block_size': block_size}
    if uuid:
        params['uuid'] = uuid
    return client.call('construct_null_bdev', params)