diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9187edf..0a0eab1 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -47,7 +47,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6
-      - uses: actions/setup-python@v5
+      - uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065  # v5
         with:
           python-version: "3.11"
       - run: python scripts/check_version_linearity.py
diff --git a/scripts/check_version_linearity.py b/scripts/check_version_linearity.py
index 6c0c765..8b61b16 100644
--- a/scripts/check_version_linearity.py
+++ b/scripts/check_version_linearity.py
@@ -28,7 +28,13 @@ def check(root: Path) -> tuple[int, str]:
     if not changelog_path.exists():
         return 1, f"CHANGELOG.md not found at {changelog_path}"
 
-    declared = tomllib.loads(pyproject_path.read_text())["project"]["version"]
+    try:
+        declared = tomllib.loads(pyproject_path.read_text())["project"]["version"]
+    except tomllib.TOMLDecodeError as exc:
+        return 1, f"invalid pyproject.toml: {exc}"
+    except (KeyError, TypeError):
+        return 1, "pyproject.toml must define [project].version"
+
     latest = latest_changelog_version(changelog_path.read_text())
 
     if latest is None:
diff --git a/tests/scripts/test_version_linearity.py b/tests/scripts/test_version_linearity.py
index d882bfc..f4c1d19 100644
--- a/tests/scripts/test_version_linearity.py
+++ b/tests/scripts/test_version_linearity.py
@@ -13,8 +13,8 @@
 def _load_script():
     """Import the script as a module (it is under scripts/, outside the package tree)."""
     spec = importlib.util.spec_from_file_location("check_version_linearity", SCRIPT)
-    module = importlib.util.module_from_spec(spec)  # type: ignore[arg-type]
     assert spec is not None and spec.loader is not None
+    module = importlib.util.module_from_spec(spec)
     spec.loader.exec_module(module)
     return module
 
@@ -79,6 +79,22 @@ def test_version_linearity_fails_when_changelog_missing(tmp_path: Path, script)
     assert "CHANGELOG.md not found" in message
 
 
+def test_version_linearity_fails_on_malformed_pyproject(tmp_path: Path, script) -> None:
+    (tmp_path / "pyproject.toml").write_text('[project\nversion = "1.0.0"\n')
+    (tmp_path / "CHANGELOG.md").write_text("# Changelog\n\n## [1.0.0] - 2026-04-28\n")
+    code, message = script.check(tmp_path)
+    assert code == 1
+    assert "invalid pyproject.toml" in message
+
+
+def test_version_linearity_fails_when_pyproject_lacks_version(tmp_path: Path, script) -> None:
+    (tmp_path / "pyproject.toml").write_text('[project]\nname = "x"\n')
+    (tmp_path / "CHANGELOG.md").write_text("# Changelog\n\n## [1.0.0] - 2026-04-28\n")
+    code, message = script.check(tmp_path)
+    assert code == 1
+    assert "[project].version" in message
+
+
 def test_latest_changelog_version_picks_first_versioned_header(script) -> None:
     text = """# Changelog