diff --git a/NaviNIBS/util/pyvista/__init__.py b/NaviNIBS/util/pyvista/__init__.py
index be153b2..bdc8ec1 100644
--- a/NaviNIBS/util/pyvista/__init__.py
+++ b/NaviNIBS/util/pyvista/__init__.py
@@ -20,7 +20,17 @@ def setActorUserTransform(actor: Actor | str, transf: np.ndarray):
 if False or isMac:
     from NaviNIBS.util.pyvista.plotting import BackgroundPlotter, PrimaryLayeredPlotter, SecondaryLayeredPlotter
     DefaultBackgroundPlotter = BackgroundPlotter
-    RemotePlotterProxy = type('__None')  # for callers to easily check if DefaultBackgroundPlotter is RemotePlotterProxy
+
+    class RemotePlotterProxy:
+        """
+        Never-instantiated stand-in so that callers can check
+        ``isinstance(plotter, RemotePlotterProxy)`` when remote plotting is unavailable.
+
+        (Note: ``type('__None')`` previously used here evaluated to ``str``, making
+        strings unintentionally satisfy such isinstance checks.)
+        """
+        pass
+
     DefaultPrimaryLayeredPlotter = PrimaryLayeredPlotter
     DefaultSecondaryLayeredPlotter = SecondaryLayeredPlotter
 else:
diff --git a/tests/test_util/test_pyvistaDefaults.py b/tests/test_util/test_pyvistaDefaults.py
new file mode 100644
index 0000000..f74682f
--- /dev/null
+++ b/tests/test_util/test_pyvistaDefaults.py
@@ -0,0 +1,23 @@
+"""
+Regression test for the RemotePlotterProxy sentinel used on platforms
+where remote plotting is unavailable (currently macOS).
+"""
+
+import platform
+
+import pytest
+
+from NaviNIBS.util.pyvista import DefaultBackgroundPlotter, RemotePlotterProxy
+
+
+@pytest.mark.skipif(platform.system() != 'Darwin',
+                    reason='sentinel branch only taken on macOS')
+def test_remotePlotterSentinelIsNotStr():
+    """
+    ``type('__None')`` evaluates to ``str``, so string values would satisfy
+    ``isinstance(x, RemotePlotterProxy)`` checks; the sentinel must be a
+    dedicated class.
+    """
+    assert RemotePlotterProxy is not str
+    assert not isinstance('some string', RemotePlotterProxy)
+    assert not isinstance(DefaultBackgroundPlotter, RemotePlotterProxy)