diff --git a/tests/domain/epc/test_glazing_overlay.py b/tests/domain/epc/test_glazing_overlay.py
index a5a6699e..9668e3aa 100644
--- a/tests/domain/epc/test_glazing_overlay.py
+++ b/tests/domain/epc/test_glazing_overlay.py
@@ -41,3 +41,12 @@ def test_glazing_types_decode_to_their_sap_codes(
     assert simulation is not None
     assert simulation.glazing is not None
     assert simulation.glazing.glazing_type == code
+
+
+@pytest.mark.parametrize("glazing_value", ["Unknown", ""])
+def test_unresolvable_glazing_produces_no_overlay(glazing_value: str) -> None:
+    # Act
+    simulation = glazing_overlay_for(glazing_value, 0)
+
+    # Assert
+    assert simulation is None