diff --git a/tests/domain/epc/test_glazing_overlay.py b/tests/domain/epc/test_glazing_overlay.py
index d65220e7..c7ba72a8 100644
--- a/tests/domain/epc/test_glazing_overlay.py
+++ b/tests/domain/epc/test_glazing_overlay.py
@@ -9,6 +9,7 @@ from __future__ import annotations
 
 import pytest
 
+from domain.epc.glazing_type import GlazingType
 from domain.epc.property_overlays.glazing_overlay import glazing_overlay_for
 from domain.modelling.scoring.overlay_applicator import apply_simulations
 from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
@@ -71,3 +72,19 @@ def test_glazing_override_remaps_every_window_and_clears_lodged_u() -> None:
     # U is cleared so the Table-24 cascade re-derives U from the new type.
     assert all(w.glazing_type == 1 for w in result.sap_windows)
     assert all(w.window_transmission_details is None for w in result.sap_windows)
+
+
+@pytest.mark.parametrize(
+    "member", [m for m in GlazingType if m is not GlazingType.UNKNOWN]
+)
+def test_every_resolvable_glazing_value_decodes_to_a_code(
+    member: GlazingType,
+) -> None:
+    # A classifier emits a GlazingType value; if the overlay can't decode it the
+    # override silently no-ops. Every non-UNKNOWN member must resolve.
+
+    # Act
+    simulation = glazing_overlay_for(member.value, 0)
+
+    # Assert
+    assert simulation is not None