我正在运行 nixos 19.09 并尝试按照创建 ruby 包的说明进行操作。但是在第一步中，我收到一个错误，即withPackages找不到该属性。
命令：
$ nix-shell -p "ruby.withPackages (ps: with ps; [ nokogiri pry ])" --show-trace
结果：
while evaluating the attribute 'buildInputs' of the derivation 'shell' at /nix/store/b6cqdsczmwxfkmc65483x7fgwdl129ar-nixpkgs-19.03.173684.c8db7a8a16e/nixpkgs/pkgs/build-support/trivial-builders.nix:7:14:
while evaluating 'getOutput' at /nix/store/b6cqdsczmwxfkmc65483x7fgwdl129ar-nixpkgs-19.03.173684.c8db7a8a16e/nixpkgs/lib/attrsets.nix:464:23, called from undefined position:
while evaluating anonymous function at /nix/store/b6cqdsczmwxfkmc65483x7fgwdl129ar-nixpkgs-19.03.173684.c8db7a8a16e/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:141:17, called from undefined position:
attribute 'withPackages' missing, at (string):1:94
我猜我以某种方式搞砸了我的 nix 商店，但我不知道如何调试它。
附加信息：
$ echo $NIX_PATH
$HOME/.nix-defexpr/channels:nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels
$ nix-channel --list
nixpkgs https://nixos.org/channels/nixos-19.03
unstable https://nixos.org/channels/nixos-unstable
$ sudo nix-channel --list
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