Building nix closure for apache fails

[ordnungswidrig]

This happens on current master (revision 0aff1ee)

vagrant@vagrant-ubuntu-trusty-64:~/nix-docker/samples$ nix-docker -t a wordpress.nix 
Temp dir /tmp/tmpYBvi9z
Building Nix closure for the image...
error: anonymous function at "/nix/store/34jk0lxmn8p8803yizqdh80a56kfhj3j-nixpkgs-15.05pre55774.b6f8d1f/nixpkgs/pkgs/servers/http/apache-httpd/2.4.nix":1:1 called with unexpected argument ‘mpm’, at "/nix/store/34jk0lxmn8p8803yizqdh80a56kfhj3j-nixpkgs-15.05pre55774.b6f8d1f/nixpkgs/lib/customisation.nix":58:12
(use ‘--show-trace’ to show detailed location information)
Traceback (most recent call last):
  File "/usr/local/bin/nix-docker", line 81, in <module>
    package_nix_path = build()
  File "/usr/local/bin/nix-docker", line 42, in build
    nix_path = subprocess.check_output(cmd)
  File "/usr/lib/python2.7/subprocess.py", line 573, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command '['nix-build', '/home/vagrant/nix-docker/nix-docker/docker.nix', '-I', 'configuration=wordpress.nix', '--arg', 'mountBuild', 'true', '--argstr', 'name', 'a', '--argstr', 'baseImage', 'busybox']' returned non-zero exit status 1