diff --git a/tests/domain/epc/test_construction_age_band_overlay.py b/tests/domain/epc/test_construction_age_band_overlay.py
index 7527b9ea..3408900b 100644
--- a/tests/domain/epc/test_construction_age_band_overlay.py
+++ b/tests/domain/epc/test_construction_age_band_overlay.py
@@ -9,7 +9,12 @@ from __future__ import annotations
 
 import pytest
 
-from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
+from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
+    EpcPropertyData,
+    SapBuildingPart,
+)
+from domain.epc.construction_age_band import ConstructionAgeBand
 from domain.epc.property_overlays.construction_age_band_overlay import (
     age_band_overlay_for,
 )
@@ -19,7 +24,9 @@ from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
 )
 
 
-def _part(epc, identifier):  # type: ignore[no-untyped-def]
+def _part(
+    epc: EpcPropertyData, identifier: BuildingPartIdentifier
+) -> SapBuildingPart:
     return next(p for p in epc.sap_building_parts if p.identifier is identifier)
 
 
@@ -84,3 +91,19 @@ def test_age_band_override_re_dates_the_main_part_only() -> None:
     assert (
         _part(result, BuildingPartIdentifier.EXTENSION_1).construction_age_band == "B"
     )
+
+
+@pytest.mark.parametrize(
+    "member", [m for m in ConstructionAgeBand if m is not ConstructionAgeBand.UNKNOWN]
+)
+def test_every_resolvable_age_band_value_decodes_to_an_overlay(
+    member: ConstructionAgeBand,
+) -> None:
+    # A classifier emits a ConstructionAgeBand value; if the overlay can't decode
+    # it the override silently no-ops. Every non-UNKNOWN member must resolve.
+
+    # Act
+    simulation = age_band_overlay_for(member.value, 0)
+
+    # Assert
+    assert simulation is not None