How can I get Pull Request details using the octokit js client?

Question

When trying to get the details for a specific pull request (via a call to ocktokit.pulls.get, I get the error "Resource not accessible by integration". Doing the exact same thing for an issue (octokit.issues.get) works as expected. I'm using the createAppAuth authorization strategy:

const octokit = new Octokit({
  authStrategy: createAppAuth,
  auth: {
    id: APP_ID,
    privateKey: PRIVATE_KEY,
    installationId: INSTALLATION_ID,
    clientId: CLIENT_ID,
    clientSecret: CLIENT_SECRET,
  },
})

Authorization can't be the issue since I can get details for a (non-PR) issue. How can I get details for a pull request?