diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 10ec411d..75736ea2 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2172,7 +2172,7 @@ class EpcPropertyDataMapper:
             # 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,
+            open_chimneys_count=schema.open_chimneys_count or 0,
             insulated_door_count=schema.insulated_door_count,
             draughtproofed_door_count=schema.draughtproofed_door_count,
             # Mechanical ventilation PCDB plumbing — feeds the §5 Table 4f