diff --git a/factory/worktree.py b/factory/worktree.py
index d0d036d06..152d85d12 100644
--- a/factory/worktree.py
+++ b/factory/worktree.py
@@ -469,7 +469,9 @@ def _bootstrap_unborn_repo(project_path: Path) -> None:
 def detect_default_branch(project_path: Path) -> str:
     """Detect the default branch for a git repository.
 
-    Cascade: remote HEAD → probe main/master → current HEAD → fallback 'main'.
+    Cascade: remote HEAD, then probe main/master, then current HEAD, then the
+    symbolic HEAD of an unborn repo. Raises RuntimeError if none of these
+    resolve, rather than guessing a branch that does not exist.
     """
     project_path = project_path.resolve()
 
@@ -524,8 +526,20 @@ def detect_default_branch(project_path: Path) -> str:
         log.debug("detect_default_branch", source="symbolic_ref", branch=branch)
         return branch
 
-    log.debug("detect_default_branch", source="fallback", branch="main")
-    return "main"
+    # Every detection method failed: no origin/HEAD, no local main or master,
+    # HEAD is detached, and the symbolic HEAD did not resolve either. Probing
+    # for main or master already happened above, so returning a hardcoded
+    # "main" here would only hand create_worktree a branch that does not
+    # exist, which then surfaces as an opaque "git rev-parse main returned
+    # non-zero exit status 128" error. Fail with something the caller can act
+    # on instead.
+    log.debug("detect_default_branch", source="fallback", branch=None)
+    raise RuntimeError(
+        f"Could not determine a default branch for {project_path}. "
+        "There is no origin/HEAD, no local 'main' or 'master' branch, and "
+        "HEAD is detached. Set `target_branch` in .factory/config.json or "
+        "check out a branch before running."
+    )
 
 
 def _list_active_worktrees(project_path: Path) -> set[str]:
diff --git a/tests/test_worktree.py b/tests/test_worktree.py
index 13f1ce58e..68ee65b19 100644
--- a/tests/test_worktree.py
+++ b/tests/test_worktree.py
@@ -352,6 +352,49 @@ def test_fallback_to_current_branch(self, tmp_path: Path) -> None:
 
         assert detect_default_branch(project) == "develop"
 
+    def test_raises_when_no_branch_detectable(self, tmp_path: Path) -> None:
+        """A detached HEAD with no main/master and no origin raises instead of
+        returning a nonexistent 'main' branch."""
+        project = tmp_path / "project"
+        project.mkdir()
+
+        env = {
+            "GIT_AUTHOR_NAME": "test",
+            "GIT_AUTHOR_EMAIL": "test@test.com",
+            "GIT_COMMITTER_NAME": "test",
+            "GIT_COMMITTER_EMAIL": "test@test.com",
+            "HOME": str(tmp_path),
+            "PATH": "/usr/bin:/bin:/usr/local/bin",
+        }
+
+        subprocess.run(
+            ["git", "init", "-b", "develop"],
+            cwd=project,
+            capture_output=True,
+            check=True,
+        )
+        (project / "README.md").write_text("hello")
+        subprocess.run(["git", "add", "."], cwd=project, capture_output=True, check=True)
+        subprocess.run(
+            ["git", "commit", "-m", "initial"],
+            cwd=project,
+            capture_output=True,
+            check=True,
+            env=env,
+        )
+        # Detach HEAD so abbrev-ref returns "HEAD" and symbolic-ref fails,
+        # leaving no detectable default branch.
+        subprocess.run(
+            ["git", "checkout", "--detach", "HEAD"],
+            cwd=project,
+            capture_output=True,
+            check=True,
+            env=env,
+        )
+
+        with pytest.raises(RuntimeError, match="Could not determine a default branch"):
+            detect_default_branch(project)
+
 
 class TestCreateWorktreeWithMaster:
     def test_create_worktree_on_master_repo(self, git_project_master: Path) -> None: