From 148d570909e9254922bcc9d348b19d9cf66a3b7d Mon Sep 17 00:00:00 2001
From: Kurt McKee <contactme@kurtmckee.org>
Date: Thu, 18 Sep 2025 15:17:54 -0500
Subject: [PATCH 1/2] Demonstrate issue #3611

---
 tests/config/cli/test_cli_ini.py | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/tests/config/cli/test_cli_ini.py b/tests/config/cli/test_cli_ini.py
index 3af81965e..c78957dbe 100644
--- a/tests/config/cli/test_cli_ini.py
+++ b/tests/config/cli/test_cli_ini.py
@@ -247,3 +247,21 @@ def test_ini_help(exhaustive_ini: Path, capfd: CaptureFixture) -> None:
     res = out.splitlines()[-1]
     msg = f"config file {str(exhaustive_ini)!r} active (changed via env var TOX_USER_CONFIG_FILE)"
     assert res == msg
+
+
+def test_ini_loader_corrupt_default_config_file(
+    mocker: MockerFixture,
+    tmp_path: Path,
+    caplog: LogCaptureFixture,
+) -> None:
+    # Setup: Create a corrupt DEFAULT_CONFIG_FILE and point to it.
+    config_file = tmp_path / "config.ini"
+    config_file.write_text("[tox\n")
+    mocker.patch("tox.config.cli.ini.DEFAULT_CONFIG_FILE", config_file)
+
+    # Act
+    IniConfig()
+
+    # Verify
+    assert "failed to read config file None" not in caplog.messages[0]
+    assert f"failed to read config file {config_file}" in caplog.messages[0]

From 95162103f2cca98278b18b9730c936d63a3e4dcd Mon Sep 17 00:00:00 2001
From: Kurt McKee <contactme@kurtmckee.org>
Date: Thu, 18 Sep 2025 15:19:09 -0500
Subject: [PATCH 2/2] Fix `failed to read config file None` when
 `DEFAULT_CONFIG_FILE` is corrupt

Fixes #3611
---
 docs/changelog/3611.bugfix.rst | 2 ++
 src/tox/config/cli/ini.py      | 2 +-
 2 files changed, 3 insertions(+), 1 deletion(-)
 create mode 100644 docs/changelog/3611.bugfix.rst

diff --git a/docs/changelog/3611.bugfix.rst b/docs/changelog/3611.bugfix.rst
new file mode 100644
index 000000000..dd0a1f72a
--- /dev/null
+++ b/docs/changelog/3611.bugfix.rst
@@ -0,0 +1,2 @@
+Fix ``None`` appearing as the config filename in error output
+when the user's default config file is corrupt. - by :user:`kurtmckee`
diff --git a/src/tox/config/cli/ini.py b/src/tox/config/cli/ini.py
index a4ee0efa9..82c29c5a9 100644
--- a/src/tox/config/cli/ini.py
+++ b/src/tox/config/cli/ini.py
@@ -39,7 +39,7 @@ def __init__(self) -> None:
                 if self.has_tox_section:
                     self.ini = IniLoader(CORE, parser, overrides=[], core_section=CORE)
             except Exception as exception:  # noqa: BLE001
-                logging.error("failed to read config file %s because %r", config_file, exception)  # noqa: TRY400
+                logging.error("failed to read config file %s because %r", self.config_file, exception)  # noqa: TRY400
                 self.has_config_file = None
 
     def get(self, key: str, of_type: type[Any]) -> Any: