auth: Optional `audience` key is invalid when unspecified

TL;DR

The docs here say audience is an optional key in the GitHub actions configuration. But when I run my action without the key specified, I get the following error:

Expected behavior

To run successfully without specifying an audience key.

Observed behavior

It says the audience key is invalid.

Action YAML

name: Deploy - Production

on:
  workflow_dispatch:
    inputs:
      deployables:
        description: 'backend/client/dispatch.yaml'
        default: 'backend client'
        required: true
      message:
        description: 'What updates are you deploying?'
        required: true

concurrency: production_environment

jobs:
  deploy:
    runs-on: ubuntu-latest
    permissions:
      id-token: write
      contents: read
    steps:
      - name: Disable client deploys
        if: contains(github.event.inputs.deployables, 'client')
        run: |
          # echo "::error::The client cannot be deployed at this time. See Slack for details."
          # exit 1

      - name: Disable backend deploys
        if: contains(github.event.inputs.deployables, 'backend')
        run: |
          # echo "::error::The backend cannot be deployed at this time. See Slack for details."
          # exit 1

      - name: Checkout code
        uses: actions/checkout@v2

      - id: 'auth'
        name: 'Authenticate to GCP'
        uses: 'google-github-actions/auth@v0.4.1'
        with:
          workload_identity_provider:
            ${{ secrets.WORKLOAD_ID_PROVIDER_PRODUCTION }}
          service_account: ${{ secrets.SERVICE_ACCOUNT_PRODUCTION }}

      - name: Set up Cloud SDK
        uses: google-github-actions/setup-gcloud@v0.3.0

      - name: Generate version tag
        run: |
          commit_hash=$(git rev-parse --short=7 HEAD)
          echo "version_tag=$(TZ=:UTC date +%Y-%m-%dt%H-%M-%Sutc-$commit_hash)" >> $GITHUB_ENV

      - name: Deploy
        run: |
          ${GITHUB_WORKSPACE}/deploy --production --message="${{ github.event.inputs.message }}" --actor="$GITHUB_ACTOR" --version=${{ env.version_tag }} ${{ github.event.inputs.deployables }}

      - name: Create Release
        uses: actions/create-release@v1
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
        with:
          tag_name: ${{ env.version_tag }}
          release_name: ${{ github.event.inputs.message }}
          draft: false
          prerelease: false

Additional information

This works fine as configured in our staging environment, which is a separate Google Cloud project.