diff --git a/tests/repositories/historic_epc/test_historic_epc_s3_repository.py b/tests/repositories/historic_epc/test_historic_epc_s3_repository.py
index 7bebcb651..2f84d7e65 100644
--- a/tests/repositories/historic_epc/test_historic_epc_s3_repository.py
+++ b/tests/repositories/historic_epc/test_historic_epc_s3_repository.py
@@ -13,6 +13,7 @@ from contextlib import contextmanager
 from unittest.mock import patch
 
 import pandas as pd
+import pytest
 from botocore.exceptions import ClientError
 
 from backend.utils.addressMatch import AddressMatch
@@ -74,3 +75,36 @@ def test_valid_postcode_with_no_stored_object_returns_empty_list():
 
     # Assert — a miss is the normal, expected outcome, not an exception.
     assert records == []
+
+
+def test_builds_s3_key_from_sanitised_postcode_and_default_root():
+    # Arrange
+    calls: list[tuple[str, str]] = []
+
+    def capture(bucket: str, key: str) -> pd.DataFrame:
+        calls.append((bucket, key))
+        return _df([_row("47 GORDON ROAD", "100")])
+
+    repo = HistoricEpcS3Repository(read_csv_gz=capture)
+
+    # Act
+    with _valid_postcode():
+        repo.get_for_postcode("ab33 8al")
+
+    # Assert
+    assert calls == [("retrofit-data-dev", "historical_epc/AB338AL/data.csv.gz")]
+
+
+def test_non_missing_read_error_propagates():
+    # Arrange — an error that is NOT a missing object must not be swallowed.
+    def denied(bucket: str, key: str) -> pd.DataFrame:
+        raise ClientError(
+            {"Error": {"Code": "AccessDenied", "Message": "nope"}}, "GetObject"
+        )
+
+    repo = HistoricEpcS3Repository(read_csv_gz=denied)
+
+    # Act / Assert
+    with _valid_postcode():
+        with pytest.raises(ClientError):
+            repo.get_for_postcode("AB33 8AL")