diff --git a/mergify_cli/stack/push.py b/mergify_cli/stack/push.py
index 75219675..033d29e8 100644
--- a/mergify_cli/stack/push.py
+++ b/mergify_cli/stack/push.py
@@ -24,6 +24,8 @@
 import sys
 import typing
 
+import rich.markup
+
 from mergify_cli import console
 from mergify_cli import console_error
 from mergify_cli import utils
@@ -582,8 +584,16 @@ async def stack_push(
             with console.status("Fetching old PR heads for comparison..."):
                 try:
                     await fetch_old_pr_heads(remote, updated_pr_numbers)
-                except utils.CommandError:
-                    pass  # Non-fatal: change type will be "unknown"
+                except utils.CommandError as exc:
+                    # Non-fatal: change type will be "unknown" — but surface
+                    # the underlying error so the user can fix it. Escape the
+                    # exception text since it can contain `[`/`]` that Rich
+                    # would otherwise interpret as markup tags.
+                    console.log(
+                        f"[orange]Could not fetch old PR heads; revision-history "
+                        f"change types will fall back to 'unknown': "
+                        f"{rich.markup.escape(str(exc))}[/]",
+                    )
 
             # Detect change types before force-push overwrites refs
             change_types: dict[str, str] = {}
diff --git a/mergify_cli/tests/test_utils.py b/mergify_cli/tests/test_utils.py
index 2114ee5c..469ba475 100644
--- a/mergify_cli/tests/test_utils.py
+++ b/mergify_cli/tests/test_utils.py
@@ -31,6 +31,13 @@
     import pathlib
 
 
+def test_command_error_str_handles_non_utf8_stdout() -> None:
+    # Some git invocations (e.g. legacy locales) can emit non-UTF-8 bytes;
+    # str(CommandError) must not raise — error paths depend on it.
+    error = utils.CommandError(("git", "show", "abc"), 1, b"\xff\xfe broken")
+    assert "failed to run `git show abc`" in str(error)
+
+
 @pytest.mark.usefixtures("_git_repo")
 async def test_get_branch_name() -> None:
     assert await utils.git_get_branch_name() == "main"
diff --git a/mergify_cli/utils.py b/mergify_cli/utils.py
index 4f5addd1..3d875a1b 100644
--- a/mergify_cli/utils.py
+++ b/mergify_cli/utils.py
@@ -92,7 +92,13 @@ class CommandError(Exception):
     stdout: bytes
 
     def __str__(self) -> str:
-        return f"failed to run `{' '.join(self.command_args)}`: {self.stdout.decode()}"
+        # ``errors="replace"`` so str(CommandError) never raises on
+        # non-UTF-8 process output — callers in error paths (warnings,
+        # CLI top-level handler) rely on this being safe.
+        return (
+            f"failed to run `{' '.join(self.command_args)}`: "
+            f"{self.stdout.decode(errors='replace')}"
+        )
 
 
 class MergifyError(click.ClickException):