diff --git a/scripts/profile_sync.py b/scripts/profile_sync.py
index 4364f8da..6478a981 100644
--- a/scripts/profile_sync.py
+++ b/scripts/profile_sync.py
@@ -2688,15 +2688,29 @@ def _set_pin(text: str, document: dict, field: str, value: str, after: str) -> s
     return insert_after_line(text, after, f'{field}: "{value}"')
 
 
-def _git_history(path: Path) -> list[str]:
-    """Commits touching the profile, newest first, empty outside a repo."""
-    result = subprocess.run(
-        ["git", "log", "--format=%H", "--", path.name],
-        cwd=path.parent, capture_output=True, text=True, check=False,
-    )
-    if result.returncode != 0:
-        return []
-    return result.stdout.split()
+def _git_history(path: Path) -> list[str] | None:
+    """Commits touching the profile, newest first; None when git cannot say.
+
+    An untracked profile has no history. A git that fails (no repository,
+    dubious ownership, git missing) has not answered, and reading that as
+    "nothing written under an older pin" let a bump through.
+    """
+    def git(*argv: str) -> subprocess.CompletedProcess:
+        return subprocess.run(
+            ["git", *argv], cwd=path.parent, capture_output=True, text=True, check=False,
+        )
+
+    try:
+        result = git("log", "--format=%H", "--", path.name)
+        if result.returncode == 0:
+            return result.stdout.split()
+        # A repository with no commit yet holds nothing written earlier.
+        inside = git("rev-parse", "--is-inside-work-tree")
+        if inside.stdout.strip() == "true" and git("rev-parse", "-q", "--verify", "HEAD").returncode:
+            return []
+    except OSError:
+        pass
+    return None
 
 
 def _git_file_at(path: Path, sha: str) -> dict | None:
@@ -2936,6 +2950,8 @@ def realign_prose(
         and document[f"{field}_commit"]
     ]
     history = _git_history(path)
+    if history is None:
+        return [f"read again: {path.name}: git history unreadable"]
     if not history:
         return []
 
@@ -2944,11 +2960,15 @@ def realign_prose(
     alive = dict.fromkeys(fields, True)
     intro: dict[str, str | None] = dict.fromkeys(fields)
     revisions: list[tuple[str, dict]] = []
+    # Fields whose writing pin lies past a revision that cannot be read:
+    # where their text came in is unknown, so they are not moved.
+    unknown: dict[str, str] = {}
     for sha in history:
         if not any(alive.values()):
             break
         past = _git_file_at(path, sha)
         if past is None:
+            unknown = {field: sha for field, live in alive.items() if live}
             break
         revisions.append((sha, past))
         values = _scalar_values(past)
@@ -2963,6 +2983,12 @@ def realign_prose(
     messages: list[str] = []
     jobs: list[tuple[Citation, dict]] = []
     for citation in citations:
+        if citation.field in unknown:
+            messages.append(
+                f"read again: {citation.field}: {citation.ref} "
+                f"(revision {unknown[citation.field][:8]} unreadable)"
+            )
+            continue
         intro_sha = intro.get(citation.field)
         if intro_sha is None:
             # The scalar is not committed yet: written now, under this pin.
diff --git a/tests/test_profile_sync.py b/tests/test_profile_sync.py
index d71e5925..d2c2d4c5 100644
--- a/tests/test_profile_sync.py
+++ b/tests/test_profile_sync.py
@@ -2283,6 +2283,9 @@ class TestWriteDryRun(unittest.TestCase):
         self.dir = Path(self.tmp.name)
         self.path = self.dir / "p.yml"
         self.path.write_text(SAMPLE, encoding="utf-8")
+        # Profiles live in a repository: git answers "no history" for an
+        # untracked file, where outside one it answers nothing at all.
+        subprocess.run(["git", "init", "-q"], cwd=self.dir, check=True)
 
     def tearDown(self):
         self.tmp.cleanup()
@@ -3589,6 +3592,23 @@ class TestRealignProse(unittest.TestCase):
         self.assertTrue(any("would recale" in m for m in messages))
         self.assertIn("(a.c:10)", self.path.read_text())
 
+    def test_an_unreadable_history_blocks_rather_than_passes(self):
+        """git failing (no repository, dubious ownership) read as "nothing
+        written under an older pin" and let a bump through."""
+        self.path.write_text(PROSE_SAMPLE, encoding="utf-8")
+        messages = profile_sync.realign_prose(self.path, self.tmp.name, dry_run=True)
+        self.assertEqual(messages, ["read again: p.yml: git history unreadable"])
+
+    def test_an_unreadable_revision_stops_the_move(self):
+        self._repo_with_advanced_pin()
+        self.files[("oldpin", "a.c")] = ["x"] * 9 + ["subject"]
+        self.files[("pin", "a.c")] = ["x"] * 13 + ["subject"]
+        with mock.patch.object(profile_sync, "_git_file_at", return_value=None):
+            messages = profile_sync.realign_prose(self.path, self.tmp.name)
+        self.assertTrue(messages)
+        self.assertTrue(all(m.startswith("read again: notes:") for m in messages))
+        self.assertIn("(a.c:10)", self.path.read_text())
+
     def test_scalar_written_at_the_current_pin_is_left_alone(self):
         self._git("init", "-q")
         self._git("config", "user.email", "t@t")