Error in first steps of Ruby guide

I’m running nixos 19.09 and I’m trying to follow along the steps in the guide to building a ruby project in nix. But I’m getting an error just trying to use nix-shell.

$ nix-shell -p "ruby.withPackages (ps: with ps; [ nokogiri pry ])" --show-trace
error: while evaluating the attribute 'buildInputs' of the derivation 'shell' at /nix/store/b6cqdsczmwxfkmc65483x7fgwdl129ar-nixpkgs-1
while evaluating 'getOutput' at /nix/store/b6cqdsczmwxfkmc65483x7fgwdl129ar-nixpkgs-19.03.173684.c8db7a8a16e/nixpkgs/lib/attrsets.nix:
while evaluating anonymous function at /nix/store/b6cqdsczmwxfkmc65483x7fgwdl129ar-nixpkgs-19.03.173684.c8db7a8a16e/nixpkgs/pkgs/stden
attribute 'withPackages' missing, at (string):1:94

I assume I messed up my environment somehow, but I’m not sure where.

As it says in your error, it’s using nixpkgs-19.03. Maybe your channels need updating? Run nix-info to find out :slight_smile: