users.extraUsers.vagrant.uid is not defined

[mmerickel]

Receiving a 404 while trying to packer build template.json.

==> virtualbox-iso: Downloading or copying ISO
    virtualbox-iso: Downloading or copying: http://releases.nixos.org/nixos/unstable/nixos-14.02pre38596.1dd0e05/nixos-minimal-14.02pre38596.1dd0e05-x86_64-linux.iso
    virtualbox-iso: Error downloading: HTTP error '404'! Remote side responded:

I updated the iso_checksum and iso_url to use the latest:

diff --git a/template.json b/template.json
index 13eabec..fab5893 100644
--- a/template.json
+++ b/template.json
@@ -25,9 +25,9 @@
       "guest_additions_mode": "disable",
       "guest_os_type": "Linux_64",
       "http_directory": "nixos",
-      "iso_checksum": "48395b4bd21675703c87cfae1feb68ed",
+      "iso_checksum": "ad16908efdcdbeff7343df77470838ee",
       "iso_checksum_type": "md5",
-      "iso_url": "http://releases.nixos.org/nixos/unstable/nixos-14.02pre38596.1dd0e05/ni
+      "iso_url": "http://releases.nixos.org/nixos/unstable/nixos-14.04pre40294.691b499/ni
       "shutdown_command": "sudo shutdown -h now",
       "ssh_username": "root",
       "ssh_key_path": "keys/vagrant.key",
@@ -49,7 +49,7 @@
   "post-processors": [
     {
       "type": "vagrant",
-      "output": "nixos-14.02-x86_64-{{.Provider}}.box"
+      "output": "nixos-14.04-x86_64-{{.Provider}}.box"
     }
   ]
 }

Unfortunately now I get an error while the system is building.

while evaluating 'id' at '/mnt-nixpkgs/lib/trivial.nix:9:8':
user-thrown exception: The option 'users.extraUsers.vagrant.uid' is used but not defined.

I apologize I don't know the process very well as I introduce myself to nix, but I wanted to do my experiments in a VM and there doesn't seem to be a publicly available vagrant box to use.