diff --git a/llvm/utils/git/github-automation.py b/llvm/utils/git/github-automation.py
--- a/llvm/utils/git/github-automation.py
+++ b/llvm/utils/git/github-automation.py
@@ -150,6 +150,10 @@
     def issue_notify_pull_request(self, pull:github.PullRequest.PullRequest) -> None:
         self.issue.create_comment('/pull-request {}#{}'.format(self.branch_repo_name, pull.number))
 
+    def issue_notify_no_milestone(self, comment) -> None:
+        message = "<!--IGNORE-->\n>{}\n\nError: Command failed due to missing milestone.".format(comment)
+        self.issue.create_comment(message)
+
     @property
     def action_url(self) -> str:
         if os.getenv('CI'):
@@ -443,6 +447,9 @@
     release_workflow = ReleaseWorkflow(args.token, args.repo, args.issue_number,
                                        args.branch_repo, args.branch_repo_token,
                                        args.llvm_project_dir)
+    if not release_workflow.release_branch_for_issue:
+        release_workflow.issue_notify_no_milestone(''.join(sys.stdin.readlines()))
+        sys.exit(1)
     if args.sub_command == 'print-release-branch':
         release_workflow.print_release_branch()
     else: