diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index 9a329a321..41cc484a1 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -452,3 +452,18 @@ def test_solid_fuel_room_heater_defaults_to_house_coal_as_its_natural_fuel() ->
     assert simulation is not None
     assert simulation.heating is not None
     assert simulation.heating.main_fuel_type == 33
+
+
+def test_air_source_heat_pump_decodes_to_the_default_ashp_code() -> None:
+    # A landlord names "air source heat pump" without a PCDB model. It is
+    # modellable via the SAP Table 4a default ASHP code (211, "flow temp in other
+    # cases", default SPF 2.30) — no main_heating_index_number needed — so it must
+    # decode to 211, not produce no overlay / fall to Unknown (ADR-0041).
+
+    # Act
+    simulation = main_heating_overlay_for("Air source heat pump", 0)
+
+    # Assert — SAP Table 4a code 211 (default air-source heat pump).
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == 211