"can't find gem bundler" with fresh install, Ubuntu

Using WSL (Windows Sybsystem for Linux) and just did a fresh install of Jekyll. I’ve got an existing site I cloned from a repo. I updated Gemfile to specify jekyll 4.0. When I try to start by: bundle exec jekyll serve --config _config.dev.yml I now get:

Traceback (most recent call last):
        2: from /home/ray/gems/bin/bundle:23:in `<main>'
        1: from /usr/lib/ruby/2.5.0/rubygems.rb:308:in `activate_bin_path'
/usr/lib/ruby/2.5.0/rubygems.rb:289:in `find_spec_for_exe': can't find gem bundler (>= 0.a) with executable bundle (Gem::GemNotFoundException)                                                                                      

I noticed in the 4.0 blog post that I should run bundle update, but that returns the same issue.

Getting rid of Gemfile.lock seems to have helped the issue.