diff --git a/__tests__/bundle/defaultBranchFallback.test.ts b/__tests__/bundle/defaultBranchFallback.test.ts
new file mode 100644
index 0000000..15565a7
--- /dev/null
+++ b/__tests__/bundle/defaultBranchFallback.test.ts
@@ -0,0 +1,113 @@
+import type { FakeGithub } from './fakeGithub'
+import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, it, vi } from 'vitest'
+
+import pullReqListPulls from '../fixtures/pullReq/pullReqListPulls.json'
+import pullReqOpenedEvent from '../fixtures/pullReq/pullReqOpenedEvent.json'
+import { start } from './fakeGithub'
+import { configReads, helpersFor, ownersProbe, ownersReads, queueRead, repo, token } from './helpers'
+import { runBundle } from './runBundle'
+
+vi.setConfig({ testTimeout: 30_000 })
+
+// the `repos.get` read `defaultBranch` falls back to when the payload names no `repository.default_branch`
+const repoRead = `GET ${repo}`
+const head = pullReqOpenedEvent.pull_request.head.sha
+
+// the pull_request payload with the base branch's name removed: `approve` then asks `repoHasOwners`
+// for the default branch instead of `branchHasOwners` for `base.ref`
+function withoutBaseRef(payload: typeof pullReqOpenedEvent) {
+  const { ref: _ref, ...base } = payload.pull_request.base
+  return { ...payload, pull_request: { ...payload.pull_request, base } }
+}
+
+// the same payload with `repository.default_branch` removed too, so the name has to come from the api
+function withoutDefaultBranch<T extends { repository: typeof pullReqOpenedEvent.repository }>(payload: T) {
+  const { default_branch: _default, ...repository } = payload.repository
+  return { ...payload, repository }
+}
+
+describe('dist/index.js: the default branch stands in when no base branch is in scope', () => {
+  let gh: FakeGithub
+  const { expectRequests, routeOwners } = helpersFor(() => gh)
+
+  beforeAll(async () => {
+    gh = await start()
+  })
+  beforeEach(() => gh.mergeQueueFallback({ pullRequestId: 'PR_none', headOid: head, enabled: false }))
+  afterEach(() => gh.reset())
+  afterAll(() => gh.close())
+
+  describe('approve on a pull_request payload without base.ref (approve.ts evaluateOnOwnersRepo → owners.ts repoHasOwners)', () => {
+    const synchronize = (payload: unknown) => runBundle({ eventName: 'pull_request', payload, inputs: token, apiUrl: gh.url })
+
+    it('probes the tree of repository.default_branch without a repos.get read', async () => {
+      routeOwners({}, ['src/file1.txt'])
+
+      const result = await synchronize({ ...withoutBaseRef(pullReqOpenedEvent), action: 'synchronize' })
+
+      expect(result.status, result.stdout).toBe(0)
+      expect(result.errors).toEqual([])
+      expect(result.stdout).toContain('approve: the base branch has no OWNERS files')
+      // owners-label's reads, then approve's probe of master (the payload's default branch); tide skips synchronize
+      expect(gh.requests.map(r => `${r.method} ${r.path}`)).toEqual([...ownersReads, ownersProbe])
+    })
+
+    it('reads the default branch from repos.get when the payload names none, then probes that tree', async () => {
+      routeOwners({}, ['src/file1.txt'])
+      gh.route('GET', repo, { status: 200, body: { default_branch: 'trunk' } })
+
+      const result = await synchronize({ ...withoutDefaultBranch(withoutBaseRef(pullReqOpenedEvent)), action: 'synchronize' })
+
+      expect(result.status, result.stdout).toBe(0)
+      expect(result.errors).toEqual([])
+      expect(result.stdout).toContain('approve: the base branch has no OWNERS files')
+      expect(gh.requests.map(r => `${r.method} ${r.path}`)).toEqual([...ownersReads, repoRead, `GET ${repo}/git/trees/trunk?recursive=true`])
+    })
+
+    it('fails the run when repos.get answers 500, probing no tree', async () => {
+      routeOwners({}, ['src/file1.txt'])
+      gh.route('GET', repo, { status: 500, body: { message: 'boom' } })
+
+      const result = await synchronize({ ...withoutDefaultBranch(withoutBaseRef(pullReqOpenedEvent)), action: 'synchronize' })
+
+      expect(result.status, result.stdout).toBe(1)
+      expect(result.errors).toHaveLength(1)
+      expect(result.errors[0]).toContain('could not read the default branch')
+      // owners-label still lists the tree at the base sha; approve probes no branch tree
+      expect(gh.requestsMatching('GET', /\/git\/trees\/(?!basesha)/)).toEqual([])
+      expect(gh.requests.map(r => `${r.method} ${r.path}`)).toEqual([...ownersReads, repoRead])
+    })
+  })
+
+  describe('tide gate on a pull request whose api read carries no base.ref (tide.ts loadTide → owners.ts repoHasOwners)', () => {
+    const pullRead = `GET ${repo}/pulls/1`
+    const bind = `POST ${repo}/statuses/${head}`
+    const bindingRead = `GET ${repo}/commits/${head}/status?per_page=100`
+    const merge = `PUT ${repo}/pulls/1/merge`
+
+    function cleanPrWithoutBaseRef() {
+      const pr = structuredClone(pullReqListPulls[0])
+      const { ref: _ref, ...base } = pr.base
+      return { ...pr, base, labels: [{ name: 'lgtm' }], number: 1, mergeable: true, mergeable_state: 'clean', head: { sha: head } }
+    }
+
+    function labeledLgtm(payload: Record<string, unknown> = pullReqOpenedEvent) {
+      return { ...payload, action: 'labeled', label: { name: 'lgtm' }, sender: pullReqOpenedEvent.sender }
+    }
+
+    const run = (payload: unknown) => runBundle({ eventName: 'pull_request', payload, inputs: { ...token, 'merge-method': 'squash' }, apiUrl: gh.url })
+
+    it('probes the tree of repository.default_branch and merges', async () => {
+      gh.commitStatuses(repo, head, [{ context: 'prow/lgtm', state: 'success' }])
+      gh.route('GET', `${repo}/pulls/1`, { status: 200, body: cleanPrWithoutBaseRef() })
+      gh.route('PUT', `${repo}/pulls/1/merge`, { status: 200, body: { merged: true } })
+
+      const result = await run(labeledLgtm())
+
+      expect(result.status, result.stdout).toBe(0)
+      expect(result.errors).toEqual([])
+      expect(result.stdout).toContain('merged pr #1')
+      expectRequests(configReads(), [bind, pullRead, ownersProbe, bindingRead, queueRead, merge])
+    })
+  })
+})