diff --git a/datatypes/epc/domain/epc_property_data.py b/datatypes/epc/domain/epc_property_data.py
index a49010d3..17437f6f 100644
--- a/datatypes/epc/domain/epc_property_data.py
+++ b/datatypes/epc/domain/epc_property_data.py
@@ -35,15 +35,18 @@ class BuildingPartIdentifier(Enum):
     OTHER = "other"
 
     @classmethod
-    def from_api_string(cls, api_identifier: Optional[str]) -> "BuildingPartIdentifier":
+    def from_api_string(
+        cls, api_identifier: Optional[object]
+    ) -> "BuildingPartIdentifier":
         """Map a gov-EPC API `BuildingPart.identifier` to its canonical
         member. "Main Dwelling" → MAIN; "Extension N" → EXTENSION_N
-        (for N in 1..4). `None` (permitted by the 21_0_1 schema) and
-        anything unrecognised fall to OTHER.
+        (for N in 1..4). `None` (permitted by the 21_0_1 schema), a non-string
+        identifier (some SAP-16.x certs lodge a bare int), and anything else
+        unrecognised fall to OTHER.
         """
         if api_identifier == "Main Dwelling":
             return cls.MAIN
-        if api_identifier is not None:
+        if isinstance(api_identifier, str):
             match = _API_EXTENSION.match(api_identifier)
             if match is not None:
                 return cls.extension(int(match.group(1)))
diff --git a/tests/datatypes/epc/domain/test_building_part_identifier.py b/tests/datatypes/epc/domain/test_building_part_identifier.py
new file mode 100644
index 00000000..d792f45a
--- /dev/null
+++ b/tests/datatypes/epc/domain/test_building_part_identifier.py
@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
+
+
+def test_main_dwelling_maps_to_main() -> None:
+    assert BuildingPartIdentifier.from_api_string("Main Dwelling") == (
+        BuildingPartIdentifier.MAIN
+    )
+
+
+def test_extension_string_maps_to_numbered_extension() -> None:
+    assert BuildingPartIdentifier.from_api_string("Extension 2") == (
+        BuildingPartIdentifier.EXTENSION_2
+    )
+
+
+def test_none_falls_to_other() -> None:
+    assert BuildingPartIdentifier.from_api_string(None) == BuildingPartIdentifier.OTHER
+
+
+def test_non_string_identifier_falls_to_other_not_crash() -> None:
+    # Some SAP-16.x certs lodge a bare int building-part identifier (e.g. the
+    # second part as `1` after "Main Dwelling"). This previously raised
+    # `TypeError: expected string or bytes-like object, got 'int'` and failed
+    # the whole property — now it falls to OTHER like any unrecognised value.
+    assert BuildingPartIdentifier.from_api_string(1) == BuildingPartIdentifier.OTHER