How to publish gem using the rubygems/release-gem action

I'm trying to publish a Ruby gem through the release-gem GitHub Action. I have copied the workflow suggested in the README:

name: publish

on:
  release:
    types: [published]

jobs:
  release:
    runs-on: ubuntu-24.04

    permissions:
      id-token: write
      contents: write

    steps:
      - name: Checkout code
        uses: actions/checkout@v4

      - name: Set up Ruby
        uses: ruby/setup-ruby@v1
        with:
          bundler-cache: true
          ruby-version: ruby

      # Publish the gem to RubyGems
      - uses: rubygems/release-gem@v1

But when it runs, I get

Running `git push origin refs/heads/HEAD` failed with the following output:

error: src refspec refs/heads/HEAD does not match any
error: failed to push some refs to 'https://github.com/inforlife/gatekeeper-api-client'

According to several questions on SO, the error is commonly related to the fact that no commit has been made. In my case, several commits have been made as the code was pushed to GitHub.

What am I missing here?