LoadError: cannot load such file -- pg_ext

[zzzo...@gmail.com]

> rake populate:users
C:/RailsInstaller/Ruby2.3.3/lib/ruby/gems/2.3.0/gems/activesupport-4.1.1/lib/active_support/values/time_zone.rb:285: warning: circular argument reference - now
rake aborted!
LoadError: cannot load such file -- 2.3/pg_ext
I:/Developer/Ruby/task/config/application.rb:7:in `<top (required)>'
I:/Developer/Ruby/task/Rakefile:4:in `<top (required)>'


Caused by:
LoadError: cannot load such file -- pg_ext
I:/Developer/Ruby/task/config/application.rb:7:in `<top (required)>'
I:/Developer/Ruby/task/Rakefile:4:in `<top (required)>'
(See full trace by running task with --trace)

I've inherited a Ruby project and am trying to set it up. I've installed Postgres, configured the `pg` gem by specifying the path to the `pg_config` program before installation, run `bundle install` w/o errors, but it fails with this message now when trying to populate the database.