building NixOS image

error

[bob@nixos:~]$ nix-build laptop.nix -A config.system.build.laptop --show-trace
error: while evaluating anonymous function at /home/bob/nixpkgs/nixos/lib/make-disk-image.nix:1:1, called from /home/bob/laptop.nix:5:32:
value is a function while a set was expected

laptop.nix

with import ./nixpkgs/lib;
with import ./nixpkgs/pkgs/top-level;

{ 
  config.system.build.laptop = import ./nixpkgs/nixos/lib/make-disk-image.nix {
    inherit lib config pkgs;
    diskSize = 4096;
  };
}