diff --git a/harness/report.py b/harness/report.py
index f138154cd..018a25469 100644
--- a/harness/report.py
+++ b/harness/report.py
@@ -196,6 +196,8 @@ def build_property_report(
     name: str = path.stem
     try:
         epc = EpcPropertyDataMapper.from_api_response(json.loads(path.read_text()))
+        if epc is None:
+            raise ValueError("Unsupported or unrecognised EPC schema")
         lodged_sap: Optional[int] = epc.energy_rating_current
         calculated_sap: float = Sap10Calculator().calculate(epc).sap_score_continuous
     except Exception as error:  # noqa: BLE001 — one bad cert must not abort the report