Pronto produces no output

Build Environment
1

on my branch locally I get output as follows:

$ bundle exec pronto run --exit-code
config/initializers/shrine.rb:4 W: Style/StringLiterals: Prefer single-quoted strings when you don't need string interpolation or special symbols.
config/initializers/shrine.rb:8 W: Style/TrailingCommaInHashLiteral: Avoid comma after the last item of a hash.
config/initializers/shrine.rb:14 W: Layout/LineLength: Line is too long. [102/80]
config/initializers/shrine.rb:15 W: Layout/LineLength: Line is too long. [110/80]
config/initializers/shrine.rb:17 W: Layout/LineLength: Line is too long. [92/80]

.circleci/config.yml

# run pronto - rubocop etc
  - run:
      name: run pronto
      command: |
        git fetch && git merge-base origin/master HEAD
        bundle exec pronto run --exit-code

ouput on circle:

#!/bin/bash -eo pipefail
git fetch && git merge-base origin/master HEAD
bundle exec pronto run --exit-code