diff --git a/package.json b/package.json
index 1d1cc89..3a6f3ec 100644
--- a/package.json
+++ b/package.json
@@ -10,7 +10,8 @@
     "@actions/github": "^5.0.0",
     "@octokit/core": "^3.5.1",
     "@octokit/plugin-request-log": "^1.0.3",
-    "@octokit/plugin-rest-endpoint-methods": "^5.13.0"
+    "@octokit/plugin-rest-endpoint-methods": "^5.13.0",
+    "@octokit/request-error": "^2.1.0"
   },
   "devDependencies": {
     "@types/node": "^14.14.25",
diff --git a/src/edit-github-blob-test.ts b/src/edit-github-blob-test.ts
new file mode 100644
index 0000000..f64362a
--- /dev/null
+++ b/src/edit-github-blob-test.ts
@@ -0,0 +1,149 @@
+import test from 'ava'
+import api from './api'
+import { Response } from 'node-fetch'
+import editGithubBlob from './edit-github-blob'
+
+type fetchOptions = {
+  method: string
+  body: string | null
+}
+
+function replyJSON(status: number, body: any): Promise<Response> {
+  return Promise.resolve(
+    new Response(JSON.stringify(body), {
+      status,
+      headers: {
+        'Content-Type': 'application/json',
+      },
+    })
+  )
+}
+
+test('edit-github-blob direct push', async (t) => {
+  const stubbedFetch = function (url: string, options: fetchOptions) {
+    function route(method: string, path: string): boolean {
+      return (
+        method.toUpperCase() === options.method.toUpperCase() &&
+        `https://api.github.com/${path}` === url
+      )
+    }
+
+    if (route('GET', 'repos/OWNER/REPO')) {
+      return replyJSON(200, {
+        default_branch: 'main',
+        permissions: { push: true },
+      })
+    } else if (route('GET', 'repos/OWNER/REPO/branches/main')) {
+      return replyJSON(200, {
+        commit: { sha: 'COMMITSHA' },
+        protected: false,
+      })
+    } else if (
+      route('GET', 'repos/OWNER/REPO/contents/formula%2Ftest.rb?ref=main')
+    ) {
+      return replyJSON(200, {
+        content: Buffer.from(`old content`).toString('base64'),
+      })
+    } else if (route('PUT', 'repos/OWNER/REPO/contents/formula%2Ftest.rb')) {
+      const payload = JSON.parse(options.body || '')
+      t.is('main', payload.branch)
+      t.is('Update formula/test.rb', payload.message)
+      t.is(
+        'OLD CONTENT',
+        Buffer.from(payload.content, 'base64').toString('utf8')
+      )
+      return replyJSON(200, {
+        commit: { html_url: 'https://github.com/OWNER/REPO/commit/NEWSHA' },
+      })
+    }
+    throw `not stubbed: ${options.method} ${url}`
+  }
+
+  const url = await editGithubBlob({
+    apiClient: api('ATOKEN', { fetch: stubbedFetch, logRequests: false }),
+    owner: 'OWNER',
+    repo: 'REPO',
+    filePath: 'formula/test.rb',
+    replace: (oldContent) => oldContent.toUpperCase(),
+  })
+  t.is('https://github.com/OWNER/REPO/commit/NEWSHA', url)
+})
+
+test('edit-github-blob via pull request', async (t) => {
+  var newBranchName: string
+  const stubbedFetch = function (url: string, options: fetchOptions) {
+    function route(method: string, path: string): boolean {
+      return (
+        method.toUpperCase() === options.method.toUpperCase() &&
+        `https://api.github.com/${path}` === url
+      )
+    }
+
+    if (route('GET', 'repos/OWNER/REPO')) {
+      return replyJSON(200, {
+        default_branch: 'main',
+        permissions: { push: false },
+      })
+    } else if (route('GET', 'repos/OWNER/REPO/branches/main')) {
+      return replyJSON(200, {
+        commit: { sha: 'COMMITSHA' },
+        protected: false,
+      })
+    } else if (route('POST', 'repos/OWNER/REPO/forks')) {
+      return replyJSON(200, {})
+    } else if (route('GET', 'user')) {
+      return replyJSON(200, { login: 'FORKOWNER' })
+    } else if (route('POST', 'repos/FORKOWNER/REPO/merge-upstream')) {
+      const payload = JSON.parse(options.body || '')
+      t.is('main', payload.branch)
+      return replyJSON(409, {})
+    } else if (route('POST', 'repos/FORKOWNER/REPO/git/refs')) {
+      const payload = JSON.parse(options.body || '')
+      t.regex(payload.ref, /^refs\/heads\/update-test\.rb-\d+$/)
+      newBranchName = payload.ref.replace('refs/heads/', '')
+      t.is('COMMITSHA', payload.sha)
+      return replyJSON(201, {})
+    } else if (
+      route(
+        'GET',
+        `repos/FORKOWNER/REPO/contents/formula%2Ftest.rb?ref=${newBranchName}`
+      )
+    ) {
+      return replyJSON(200, {
+        content: Buffer.from(`old content`).toString('base64'),
+      })
+    } else if (
+      route('PUT', 'repos/FORKOWNER/REPO/contents/formula%2Ftest.rb')
+    ) {
+      const payload = JSON.parse(options.body || '')
+      t.is(newBranchName, payload.branch)
+      t.is('Update formula/test.rb', payload.message)
+      t.is(
+        'OLD CONTENT',
+        Buffer.from(payload.content, 'base64').toString('utf8')
+      )
+      return replyJSON(200, {
+        commit: { html_url: 'https://github.com/OWNER/REPO/commit/NEWSHA' },
+      })
+    } else if (route('POST', 'repos/OWNER/REPO/pulls')) {
+      const payload = JSON.parse(options.body || '')
+      t.is('main', payload.base)
+      t.is(`FORKOWNER:${newBranchName}`, payload.head)
+      t.is('Update formula/test.rb', payload.title)
+      t.is('', payload.body)
+      return replyJSON(201, {
+        html_url: 'https://github.com/OWNER/REPO/pull/123',
+      })
+    }
+    throw `not stubbed: ${options.method} ${url}`
+  }
+
+  const url = await editGithubBlob({
+    apiClient: api('ATOKEN', { fetch: stubbedFetch, logRequests: false }),
+    owner: 'OWNER',
+    repo: 'REPO',
+    filePath: 'formula/test.rb',
+    replace: (oldContent) => oldContent.toUpperCase(),
+  })
+  t.is('https://github.com/OWNER/REPO/pull/123', url)
+})
diff --git a/src/edit-github-blob.ts b/src/edit-github-blob.ts
index 0957c56..d330405 100644
--- a/src/edit-github-blob.ts
+++ b/src/edit-github-blob.ts
@@ -1,4 +1,5 @@
 import type { API } from './api'
+import { RequestError } from '@octokit/request-error'
 import { basename } from 'path'
 
 async function retry<T>(
@@ -67,10 +68,18 @@ export default async function (params: Options): Promise<string> {
     const timestamp = Math.round(Date.now() / 1000)
     headBranch = `update-${basename(filePath)}-${timestamp}`
     if (needsFork) {
-      await api.repos.mergeUpstream({
-        ...headRepo,
-        branch: repoRes.data.default_branch,
-      })
+      try {
+        await api.repos.mergeUpstream({
+          ...headRepo,
+          branch: repoRes.data.default_branch,
+        })
+      } catch (err) {
+        if (err instanceof RequestError && err.status === 409) {
+          // ignore
+        } else {
+          throw err
+        }
+      }
     }
     await retry(needsFork ? 6 : 0, 5000, async () => {
       await api.git.createRef({