diff --git a/mergify_cli/__init__.py b/mergify_cli/__init__.py
index 2d715f9..dcc0194 100755
--- a/mergify_cli/__init__.py
+++ b/mergify_cli/__init__.py
@@ -390,7 +390,7 @@ async def log_httpx_response(response: httpx.Response) -> None:
     )
 
 
-def get_trunk(trunk: str | None = None) -> str:
+def get_trunk() -> str:
     try:
         trunk = subprocess.check_output(
             "git config --get mergify-cli.stack-trunk",
@@ -401,9 +401,13 @@ def get_trunk(trunk: str | None = None) -> str:
         trunk = ""
 
     if not trunk:
-        dest_branch = subprocess.check_output(
-            "git rev-parse --abbrev-ref HEAD", shell=True, text=True
-        ).strip()
+        try:
+            dest_branch = subprocess.check_output(
+                "git rev-parse --abbrev-ref HEAD", shell=True, text=True
+            ).strip()
+        except subprocess.CalledProcessError:
+            return ""
+
         try:
             trunk = subprocess.check_output(
                 f"git for-each-ref --format='%(upstream:short)' refs/heads/{dest_branch}",
@@ -636,8 +640,7 @@ async def stack_main(args: argparse.Namespace) -> None:
     )
 
 
-def cli() -> None:
-    global DEBUG
+def parse_args(args: typing.Sequence[str]) -> argparse.Namespace:
     parser = argparse.ArgumentParser()
     parser.add_argument("--debug", action="store_true", help="debug mode")
     parser.add_argument(
@@ -687,12 +690,18 @@ def cli() -> None:
         help="branch prefix used to create stacked PR",
     )
 
-    known_args, extra = parser.parse_known_args()
+    known_args, _ = parser.parse_known_args(args)
     if known_args.action is None:
         sys.argv.insert(1, "stack")
 
-    args = parser.parse_args()
+    return parser.parse_args(args)
+
+
+def cli() -> None:
+    args = parse_args(sys.argv[1:])
+
     if args.debug:
+        global DEBUG
         DEBUG = True
 
     asyncio.run(args.func(args))
diff --git a/mergify_cli/tests/test_mergify_cli.py b/mergify_cli/tests/test_mergify_cli.py
index eb8da25..3410863 100644
--- a/mergify_cli/tests/test_mergify_cli.py
+++ b/mergify_cli/tests/test_mergify_cli.py
@@ -1,5 +1,5 @@
 #
-#  Copyright © 2021 Mergify SAS
+#  Copyright © 2021-2023 Mergify SAS
 #
 # Licensed under the Apache License, Version 2.0 (the "License"); you may
 # not use this file except in compliance with the License. You may obtain
@@ -13,6 +13,23 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import subprocess
+from unittest import mock
 
-def test_noop() -> None:
-    pass
+import pytest
+
+import mergify_cli
+
+
+def test_cli_help(capsys: pytest.CaptureFixture[str]) -> None:
+    with pytest.raises(SystemExit, match="0"):
+        with mock.patch(
+            "subprocess.check_output",
+            side_effect=subprocess.CalledProcessError(2, ""),
+        ):
+            mergify_cli.parse_args(["--help"])
+
+    stdout = capsys.readouterr().out
+    assert "usage: " in stdout
+    assert "positional arguments:" in stdout
+    assert "options:" in stdout