diff --git a/tests/domain/epc/test_main_fuel_overlay.py b/tests/domain/epc/test_main_fuel_overlay.py
index 3f6bfdeb..1b554f5b 100644
--- a/tests/domain/epc/test_main_fuel_overlay.py
+++ b/tests/domain/epc/test_main_fuel_overlay.py
@@ -50,3 +50,12 @@ def test_community_mains_gas_is_a_distinct_fuel_code() -> None:
     assert simulation is not None
     assert simulation.heating is not None
     assert simulation.heating.main_fuel_type == 20
+
+
+@pytest.mark.parametrize("main_fuel_value", ["Unknown", "", "no heating or hot water"])
+def test_unresolvable_fuel_produces_no_overlay(main_fuel_value: str) -> None:
+    # Act
+    simulation = fuel_overlay_for(main_fuel_value, 0)
+
+    # Assert
+    assert simulation is None