Can't create a slack message from diff between 2 branches

Hey all,
I’m using slack orb to create a deployment message that will be sent to a channel on success.
My job is all set and I was able to actually send a message, but when adding the git diff command I always get this error:
fatal: ambiguous argument 'prod..main': unknown revision or path not in the working tree.

here’s how my job looks like:

      - image: 'cimg/base:stable'
      - checkout
      - run:
          name: Checkout branches and generate commit message
          command: |
            git clone $CIRCLE_REPOSITORY_URL myrepo
            cd myrepo
            git checkout main
            git pull
            git checkout prod
            git pull
            echo 'export COMMIT_MESSAGE="$(git log --pretty=format:"%s (%ad)" --date=short prod..main)"' >> $BASH_ENV
      - slack/notify:
          event: pass
          channel: $SLACK_LINGO_CHANNEL
          custom: //custom blocks with $COMMIT_MESSAGE in it

so what am I doing wrong here? shouldn’t the docker environment be able to see the diff between the two branches? would appreciate any help, thanks!