diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 2f2bd00b..1658383f 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2150,7 +2150,10 @@ class EpcPropertyDataMapper:
             door_count=schema.door_count,
             habitable_rooms_count=schema.habitable_room_count,
             heated_rooms_count=schema.heated_room_count,
-            wet_rooms_count=schema.wet_rooms_count,
+            # 21.0.1 lodges wet_rooms_count as Optional; None violates the
+            # epc_property NOT-NULL column (and the calc's `> 0` check). Coalesce
+            # to 0 like every other mapper (RdSAP "not lodged" → calc minimum 1).
+            wet_rooms_count=schema.wet_rooms_count or 0,
             extensions_count=schema.extensions_count,
             open_chimneys_count=schema.open_chimneys_count,
             insulated_door_count=schema.insulated_door_count,
diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index e0c6b1ba..023f7c60 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -379,6 +379,22 @@ class TestFromRdSapSchema21_0_1:
 
     # --- general ---
 
+    def test_omitted_wet_rooms_count_defaults_to_zero_not_none(self) -> None:
+        # 21.0.1 lodges wet_rooms_count as Optional, so a cert that omits it
+        # mapped to EpcPropertyData.wet_rooms_count=None. That None then
+        # violated the epc_property.wet_rooms_count NOT-NULL constraint when a
+        # predicted EPC (which deep-copies a comparable template) was persisted,
+        # and would crash the calc's `wet_rooms_count > 0` check. 37 modelling_e2e
+        # properties failed on the 2026-06-23 run for this reason. Default to 0,
+        # matching every other mapper (RdSAP 0 → calc's Table-?? minimum 1 fan).
+        data = load("21_0_1.json")
+        data.pop("wet_rooms_count", None)
+        schema = from_dict(RdSapSchema21_0_1, data)
+
+        result = EpcPropertyDataMapper.from_rdsap_schema_21_0_1(schema)
+
+        assert result.wet_rooms_count == 0
+
     def test_uprn(self, result: EpcPropertyData) -> None:
         assert result.uprn == 12457