diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index dff74bccf..266e14415 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -6,6 +6,8 @@ calculator reads from the primary system; the overlay is whole-dwelling.
 
 from __future__ import annotations
 
+import logging
+
 import pytest
 
 from domain.epc.property_overrides.main_heating_system_type import MainHeatingSystemType
@@ -90,6 +92,22 @@ def test_electric_room_heaters_overlay_the_room_heater_charge_control() -> None:
     assert simulation.heating.main_heating_control == 2601
 
 
+def test_overlay_logs_when_it_cannot_complete_a_companion_set(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    # An archetype the overlay cannot yet fully companion — here an air-source
+    # heat pump, whose Table 4e Group 2 conservative control is not yet mapped —
+    # is still overlaid (log-and-continue), but the incomplete companion set is
+    # logged as an error so the known gap is visible (CloudWatch) rather than
+    # silently inheriting the replaced system's stale value.
+    # Act
+    with caplog.at_level(logging.ERROR):
+        main_heating_overlay_for("Air source heat pump", 0)
+
+    # Assert — the gap is reported and names the archetype.
+    assert "Air source heat pump" in caplog.text
+
+
 @pytest.mark.parametrize(
     ("main_heating_value", "code"),
     [