I need help packaging pru, a Ruby app

Hello, people!

I have followed the Nixpkgs manual on packaging Ruby apps and wrote the expression below:

However, when I try to run an example like ls -1 ~ | pru size, it barfs with a message:

Traceback (most recent call last):
	3: from ./result_new-pru-pru/bin/pru:19:in `<main>'
	2: from /nix/store/wqxpaf6f373n5nrhdxf6k05h7ya75k3y-bundler-2.2.24/lib/ruby/gems/2.7.0/gems/bundler-2.2.24/lib/bundler.rb:149:in `setup'
	1: from /nix/store/wqxpaf6f373n5nrhdxf6k05h7ya75k3y-bundler-2.2.24/lib/ruby/gems/2.7.0/gems/bundler-2.2.24/lib/bundler/runtime.rb:13:in `setup'
/nix/store/wqxpaf6f373n5nrhdxf6k05h7ya75k3y-bundler-2.2.24/lib/ruby/gems/2.7.0/gems/bundler-2.2.24/lib/bundler/definition.rb:426:in `ensure_equivalent_gemfile_and_lockfile': You are trying to install in deployment mode after changing (Bundler::ProductionError)
your Gemfile. Run `bundle install` elsewhere and add the
updated ../../nix/store/mysfi2mp67q66d5xz4gps1zmnibsvn5b-gemfile-and-lockfile/Gemfile.lock to version control.

If this is a development machine, remove the /nix/store/mysfi2mp67q66d5xz4gps1zmnibsvn5b-gemfile-and-lockfile/Gemfile freeze 
by running ``.

The list of sources changed

You have added to the Gemfile:
* source: locally installed gems

Can someone help?

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