diff --git a/tests-unit/compare_test/conftest_test.py b/tests-unit/compare_test/conftest_test.py
new file mode 100644
index 000000000..01b19a942
--- /dev/null
+++ b/tests-unit/compare_test/conftest_test.py
@@ -0,0 +1,22 @@
+import os
+import tempfile
+
+import pytest
+
+from tests.compare.conftest import gather_file_basenames
+
+
+class TestGatherFileBasenames:
+    def test_returns_png_basenames(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            open(os.path.join(tmpdir, "a.png"), "w").close()
+            open(os.path.join(tmpdir, "b.txt"), "w").close()
+            open(os.path.join(tmpdir, "c.png"), "w").close()
+            assert sorted(gather_file_basenames(tmpdir)) == ["a.png", "c.png"]
+
+    def test_missing_directory_returns_empty_list(self):
+        assert gather_file_basenames("/nonexistent/path/12345") == []
+
+    def test_non_string_input_raises(self):
+        with pytest.raises(AssertionError):
+            gather_file_basenames(123)  # type: ignore[arg-type]
diff --git a/tests/compare/conftest.py b/tests/compare/conftest.py
index dd5078c9e..e2368e4f5 100644
--- a/tests/compare/conftest.py
+++ b/tests/compare/conftest.py
@@ -27,12 +27,11 @@ def args_pytest(pytestconfig):
     return args
 
 
-def gather_file_basenames(directory: str):
-    files = []
-    for file in os.listdir(directory):
-        if file.endswith(".png"):
-            files.append(file)
-    return files
+def gather_file_basenames(directory: str) -> list[str]:
+    assert isinstance(directory, str), f"directory must be str, got {type(directory).__name__}"
+    if not os.path.isdir(directory):
+        return []
+    return [file for file in os.listdir(directory) if file.endswith(".png")]
 
 # Creates the list of baseline file names to use as a fixture
 def pytest_generate_tests(metafunc):